Don't cross the 2 streams!  

The M7 bayonet makes for a decent field knife by itself, but being able to mount it as a bayonet obviously adds to it's utility.  There are add-on bayonet lugs available.  I doubt whether many bayonet charges will be made in the future, but I can say, for a certainty that a bayonet is very intimidating as a crowd-control device.   I've been on the wrong end of a line of troops with fixed bayonets, and it's not something I'll ever forget.

For a means of crowd control, and prisoner control, the bayonet remains an intimidating and useful tool.

However, the M7 and its' brothers are good field knives, and not impossibly heavy for such duty.  The M7 is heavier than the usual Mora Knife, but the Mora, while a more practical knife,  cannot be used as a bayonet. 

Within certain parameters, yes.  The muzzle device on the barrel  (flash suppressor or comp) or the Outer Diameter of the barrel itself, must accommodate the mounting ring of the Bayonet.    Assuming this is so, then there must be a lug affixed to the barrel or front sight base, which will allow the rear of the bayonet to lock-in.  There are many aftermarket Bayo lugs available.

Original bayo set-up was 20"bbl and front sight with bayo lug.  It has become fashionable to construct very short-barreled rifles nowadays.  Very few, if any, of them are designed to accommodate a bayonet.

In some cases, true.  Hence my mentioning the M-10 bayonet scabbard above.

Doesn't matter if the blade is facing up or down, which is to say that the blade is oriented vertically; what matters is that the blade is facing the wrong way: it should be oriented horizontally for use on humans.

Some history.  The bayonet was not only used in infantry charges against opponents; it was also used to defend against cavalry charges.  The front rank would kneel and project the bayonet tipped rifle upwards at an angle, and the second rank would stand in between them also with bayonets affixed and raised.  Believe it or not, horses could see the shiny, pointy things as they got close, and would very often balk and shy away from the wall of shiny metal bayonets.  British Square Receiving Cavalry Charge at Waterloo.  That's one reason the bayonet was issued in the white, even up through WW I, when cavalry charges were still believed to be possible--  So the horses could more easily SEE it.

Horses have their rib cages with their ribs almost always vertically oriented, and that is how the current bayonet is oriented.  However, the Human Rib Cage has its ribs oriented horizontally.  I expect that some bayonet jams were because the wrongly-oriented bayonet got stuck between the human opponent's ribs; such a thing would be far less likely to occur if stabbing a horse with the animal-oriented bayonet.

With the disappearance of cavalry charges, it was decided to shorten the knife to a lighter, more field-useable design, and presumably this is the reason why the blade has remained in its' current, vertically-oriented, edge down configuration.  The blades were darkened, because it was no longer necessary to intimidate horses, and because camouflage.

However, with the advent of body armor, it may be time to re-think the bayonet.  While the common spear-tipped bayonet/field knife is probably useless against hard armor, there is considerable real estate on the armored human opponent clad only in soft armor.  If the modern bayonet becomes a fighting tool in the future, perhaps we will revert to a round, triangular or cruciform spike-style bayonet, which probably stands a better chance of penetrating soft armor, and is most unlikely to get stuck.  Round Spike Bayonet.  Examples of the triangular style bayonet are those used by Brits and Americans up through US CivWar, and the cruciform bayonet is typified by the French Lebel and Chinese SKS bayonets.  Who knows, but what was once old may become new again.