Given the tedious natue of drilling-out a steel bolt from a soft aluminium casting, and having to do that for two and possibly three bolts.

And, given that this is a new car to you and you don't know the exact maintenance on the oil trap assembly (a known weak spot and maintenance need on these cars).

Perhaps the best thing to do is to remove the manifold, take care of the flame trap, and replace the manifold with one from a boneyard. There are a million of these manifold in boneyards now and with the practice you have in pulling your's off first, you will probably be able to get a salvaged one out in a half-hour, and not have to deal with the issues of machine work with your existing manifold.