Sounds like the trigger parts have an undersized diameter. Don’t sand down the pins, as they need to lock into the receiver on the right side…
And probably your grip screw is just a little too long. You don’t need a T nut in a milled receiver, it is built in/riveted onto the receiver already.
You can probably just trim down the grip screw if it is hitting the trigger parts, take a look to see how far it is protruding into the interior and how much can be trimmed down. Just don’t fuck up the threads when doing it. If you re-used the grip screw from the thumbhole stock it is most certainly not the correct length for a regular grip.
I would use a micrometer and figure out what the deal is on the pin not fitting into the trigger group. Not familiar with the design, so hard to say what the deal is, but it most definitely it is not the pins, so don’t fuck with them. You could use a micrometer to check the parts, or pin gauges, and see how close things are or not.