I think you've put some logical thought into the thing, and it shows.

I could quibble with some things, but they would be personal preferene issues, not terribly substantial.

One notes that your kit does not provide for a sidearm plus its ammo, which is something that you might want to address.  You might also want to look into small radio comm gear, as while you are away from base, you might need to contact them, or others in your group.

Sure, I have the time, but you seem to have done a pretty good job of prioritizing your requirements, and researching the means to meet them.  I really have nothing substantial to add.

I presume at least one of those nalgene carriers contains both bottle and cup?  It's possible that you might want to boil some water at some point, and the cup being present will make that possible.

When you say "first-line", that sort of implies that the above rig will often be used in conjunction with "first-line" no?  If so, then the only comment I would add is to try to obtain such items as will be easily transferred back-and-forth between the two sets of gear, as needed.  Of course, you will take care that the two rigs interface as seamlessly as possible.
It will be interesting to see the two together, and how they work in conjunction with each other.

One possible solution to your hydration device issue is to use a Camelbak carrier with attached shoulder straps.  Clip the carrier to the outside of the main pack for travel––or put it inside the pack or under the top flap.  When doffing the main pack, the Camelbak can be worn with 1/2 line gear via its' own shoulder straps.  Nalgenes can go inside main pack as reserve water.   One other trivial ––or maybe not so trivial point–– Might want to lower your Line 2 rig, at least the front, once you get Line 1 in place.  The lower the mags, the easier the access.  You will want wide, flat––maybe just wide webbing––shoulder straps on line 1 belt, lest things start stacking up on your shoulders, and causing discomfort.

Just some musings, but perhaps some of you all might provide some illumination on the subject.

First, let's do some review.

1/Line gear: Items that never leave your person, unless for extremely brief intervals, and not many of them.  This includes, but is not limited to hydration, personal defense, immediate first-aid, navigation, and small amount of food.

2/Line gear:  This also augments the above, but the main purpose is fighting.  Included, but not limited to mags/carriers, immediate need weapons tools/cleaning kit.

3/Line gear:  Everything else needed to survive/accomplish mission.

Obviously, there can be overlap between the lines, and some reasonable measures can be made to accomplish one's goals/mission.

Here's the problem;  We have 3 lines of gear, but only so much room on the body to fit all this stuff.

The point is that the user, either out of necessity or for personal conveniance, can shed the layers without adverse effect on the layers still retained/worn.  IOW, dumping the backpack should have no negative short-term effects as long as the user still retains his L1/L2 gear.

If the user will never, ever have to wear a backpack with a waistbelt––and this precludes all but the most modest-sized backpacks–– then the user can wear a vest rig above (L2) the (misnamed) "Battle Belt" which is really an L1 carrier.

The wearing of any decent, modern packpack (L3) necessarily requires room on the user's hips for the hip-belt.  This is OK, if the user wears a vest that has enough real estate to accomodate both line L1/L2 items.  This approach would seem to require an extended pistol adapter which was thin enough to be worn beneath the backpack waistbelt, but was still attached to the bottom edge of the L1/L2 vest, which was raised sufficiently to clear the pack waist belt.

In short, you can wear a chest rig, rigged high to accomodate waistbelt of L3 backpack (in which case your chest rig should include L1 items),  OR you can wear chest rig rigged high to accomodate L1 waistbelt.  Can't do both.  And there's the problem.

Note that I exclude leg-mounted platforms.  In my experience, these are problematic for a number of reasons.  For one, any weight attached to one's legs will slow one down in running, as the weight has to be not only carried, but moved back and forth in a reciprocating motion.  Very inefficient, and tiring.  Some other issues include snagging, and construction issues that interfere with L3 gear, such as improperly-placed buckles/inadequate length of extension so as to clear L3 belt.  Small, lightweight leg platforms have their uses, but any significant weight/bulk should be avoided, IMHO.

With all the above said, and with a nod to those users whose needs differ, my thinking at this point, is along the following lines, based on what I possess currently:

Line 3:  Either MOLLE II (updated) backpack, CFP-90 (updated), or Large ALICE Hellcat. (updated).  All of these packs would be capable of carrying a smallish clip-on parasite pack that would be quickly detached, and bridge the gap between L2/L3 for 2-3 days.

Line 2/1:  Molle II vest or ILBE Medic's vest.  This would have enough real-estate to carry fighting/existence load, especially when augmented by parasite pack from main pack.

Side note:  Not a disparagement on other's approaches.  Under certain circumstances, my "take' on things can/would be all wrong, and other's "take" be much better.

This post is made in an attempt to provoke discussion on how the 3 Lines of gear interface with each other, and how best to achieve our goals.

Last word:  As a Long-time collector of milsurp, I have ensembles dating from SpanAm war to present, and have been a RevWar-CivWar re-enactor as well as an occasional camper.  I've used most of this stuff, but not in combat.  In addition, I could probably outfit a rifle Team with most equipment, aside from weapons.  FWIW, I've got so much milsurp gear that my problem is NOT having equipment, but deciding on what to choose.  No brag, just fact.  OTOH, doesn't make me a guru, just a long-term collector with a tactical, utilitarian bent .

Your constructive criticism is most welcome

I would add that the issue of shoulder straps is somewhat under-recognized.  It is all well and good to wear most anything if it is stand-alone, but if one layers things, the thickness of the lower layers can be an issue.

For example, it would be uncomfortable to wear thick, ALICE-sized shoulder pads underneath a plate carrier, or Interceptor vest.  In such a case, the shoulder straps beneath the Interceptor vest should be wide (1-1/4", 1/1/2", or 2") webbing, and the width of the webbing is not only dependent on the load being carried, but also the mounting points of such webbing on the belt, and whether or not the configuration is straight-up over shoulder (giving more lee-way), or "X" harness, which is easier to don.

IMHO, users should give some thought to the thickness of their shoulder straps for lower (L1/L2) gear, lest something that is satisfactory as a stand-alone interfere with something that will go on top of it.  

It is also worth adding that the user should don the heaviest possible clothing––usually cold-wet gear–– and don gear after adjusting for the clothing.  It is not unknown for large persons, wearing bulky clothing, to run short of adjustment on their gear, which is usually sized for young, fit males.  Check it out beforehand.  Usually, it is only necessary to add-on some additional strapping, possibly (but not always) requiring some sewing.  If sewing is needed, better to find out beforehand in your garage, then when you really need the thing right away.

I also understand that some users do not, for a variety of reasons, anticipate a need to 'stack-up" all 3 lines of gear.  One hopes your plans do not fail you, but if they do, you might want to consider tailoring your gear to allow you the widest possible array of options.  I think we can all agree that it is best to have the widest array of options, and to not self-impose constraints.  Not a slam on anyone, by any means, just a suggestion/food for thought.

As an aside, I'm having a local cobbler revamp the shoulder-straps of a GI MOLLE I style vest belt.  As originally configured, the shoulder-straps, which originate on the middle, rear portion of the 3-part belt terminated at top of shoulder, which was too short.  Mod consists of cutting OEM straps at below shoulder blade length on user's back, and adding lengths of webbing that extend over shoulder to users pectorals.  Such an added length will allow no-sew fastening to 1" straps coming up from right and left front portions of the belt, with a great deal of adjustability for clothing and such. Cutting and stitching-in below shoulder blades reduces possibility of interferance/comfort issues if pack or similar is worn over the shoulder straps of the vest.  In addition, and if desired, the rear, middle portion of the belt can be unbuckled from the front 2 portions of the belt, and return to its original home, a MOLLE I vest, with no loss of functionality.  Also adding 2 "D" rings on lower back, which will allow attachment of ILBV-type GI buttpack.  I know, I said I hated 'em, but recall that I also said that the ILBV-type buttpack was the only decent one, as it could be attachec/detached without too much fuss, unlike the earlier versions which mandated reconfiguring the rear of the harness as well as the attaching/detaching operation.

Lots of intelligent thoughts in this thread.  Might have to look into making sure it's not archived and always available...