I sealed the gas block to the barrel with some loctite 620 and some blue around the roll pin and adjuster screw, and it seems to have resolved the issue--LRBHO working on several different types of ammo, across several different 2-round mags (before it would only run the first few then stop locking open).
The gas port on this Criterion barrel is definitely small, but it can be made to work with a lighter buffer, and the smaller size lets it cycle smoother, so being small isn't entirely bad. However there are other arguments in favor of drilling it larger (subsonic ammo, for example), and I am going to do that, but I will make another thread for that discussion.
To answer the original query: Forward Controls 7.62 ABC/R is the preferred bolt catch--its got the best machining and finish, and the fact that it will not catch the BCG body means that its a simple pass/fail observation for testing (it either locks open on the bolt, or it closes all the way).
Thanks for the comments.