Good update, and good job, man.
Yeah, those 4-40 set screws - you have to shorten the spring the same length as the screw, and if you force it all together, there will be so much spring pressure on the detent that you'll never get that takedown pin to move. Another side to that - if you feel it's really tough to get that takedown pin going - pull the spring, clip off another coil, and try it out. Still tight? Cut another coil, etc.
I use 6-32 set screws, too. You just have to ream the hole size up a little, then tap it. If you have the 4-40 tap,you can just run it (tap) the standard hole straight away, no reaming required. We've been doing this for 10 years here for the takedown pin, before lowers were ever threaded for that. Aero was actually the first company to start offering that, and it was on their AR15 lowers.