I'm in the same boat OP.
Just bought a colt m16a1, and plan to shoot 9mm out of it.
What you describe is my plan as well. Top drop in Hahn adapter. As far as I know you do not need to remove the Bolt catch to drop it in for the non dedicated mag block(it's simply drop in). And I don't want a tension screw pressing on my $25k lower receivers magwell. If your removing the upper receiver to swap uppers/calibers, removing the magblock is a simple part of the process.
Based on what I've read....DO NOT use the last round bolt hold open block with your m16 lower. It'll cause unnecessary damage to your bolt catch ears on the lower itself.
Not that I've heard of this being a problem, but in theory, there should be even less problems with a top drop in, since it can't slide down at all if the tension isn't tight enough. Like I said, I haven't really heard of this issue, but with the top drop in, it can't happen whatsoever.