Do you know where it says you can't use Antimagic on an instant/momentary spell? I'm not necessarily disagreeing with you - I recall reading this as well - but I'm not finding it in the core rulebook. My thought is that while, yes, you can't necessarily cast one and then the other, there could potentially be other timing options - you could potentially build the Masking effect into a device that automatically triggered each time you cast a spell, for example. I wanted to find the actual text.
Can you cite a ruling on this second one? Because that seems to completely defeat the purpose of Masking the Odor of Magic if the masking spell itself can be detected. I mean, yes - it can be detected by an InVi spell of 2x its own level - but once the base effect of Masking is over 25, the only way to detect it would be via a lvl 50+ effect, which can't be worked into a device. (which is what would be making the counter attempt).