When to apply failure to maintain game state
Cool that you are going to judge your first Competitive REL event soon, enjoy it!
FtMGS penalties are applied when your opponent commits a Game Play Error that you could have noticed, but still let it happen (unintentionally).
Both examples you give here are a GPE - Game Rule Violation by the opponent, so the other player will get a Warning for FtMGS in both cases. In Example B, the Brainstorm player resolved the Brainstorm partially and thus incorrectly, which the othe rplayer should and could notice, so he gets the according penalty.
I'm very curious why your mentor thought that example B is different than example A!
Greetz, Dustin.