Have this mod on my rifle, but now I’m wondering why I had to replace the bolt catch. Anyone know if the normal paddle-style one would still work? And if so, are there any issues, legal or otherwise, in using that instead of the one they sent me?

I don't have this mod.  I believe the replacement mag catch takes up the space used by the original bolt release.  Likewise, the bolt is allowed to move forward on the empty mag because the upper/lower won't separate with the bolt locked back.  One would need to release the bolt forward before opening and mag change and then cycle the bolt again so this eliminates the need to release the bolt before the mag change.

Ed.  Looking further, the BCG remains forward of the buffer in the upper, the buffer remains in the tube in the lower so the receiver halves separate.  Duh.  Brain dead.

I have one of these.  The replacement part no longer functions as a bolt catch, but as a magazine lock.  When the upper is in the normal position, it pushes down on the spring-loaded lock and prevents movement of the magazine catch.  This makes the rifle compliant as a magazine-locked semi auto.

I installed these on two of my AR's and I don't care for them. They work for their intended purpose but I don't like that it doesn't go to bolt lock on the last round.

Seems better than non-featured fun.  What is yiur solution?

I might go with the fin... I hate to do it but I'm considering it.

I've tried finned grips but do not care for them.  They're not bad but not what I prefer.

I've tried a stumpy, horizontal grip but didn't care for it, at all.

I am currently using a Thordsen stock on a featureless rifle.  

The Thordsen is the best, so far.  It fits well, is ergonomically correct and the rifle operates normally in every way.  

Featureless means I can use standard capacity, detachable magazines.  It is sad to say, living here in AMerica, that in this time of mob violence and mayhem and the complete lack of police enforcement, that seems like a distinct NEED when considering my self defense options.