including/some of which were

[Whit Garner]

I hear you on the rabbit hole - you definitely found one! And good news, the GMAT is never going to require you to be this nuanced in finding a right answer. That said, here are a couple of notes for your questions:

Does the insertion of the adverbial modifier “during his first term” cause issues for the modification of “several posts” by the ‘Subgroup Modifier’ (“among them X and Y”)?

When you have adverbial modifiers (and even noun modifiers) offset by a comma, you're given a lot of flexibility for intervening modifiers. The idea is more around which are more essential!

Hope this helps!