It's not venting out-of-game frustration, I can garuntee that much. yes, Remilia's pissing me off, but this has been in my head ever since the first time someone assumed Bhava-Agra was attacking. Remilia's just the only one we've seen keep to that belief; all the others we've met more than once seem to at least accept that it's possible the islands falling wasn't part of an attack (although we haven't run into Marisa again, so she's uncertain).
As for what I'm planning to say, you got fairly close to the idea when you accused Remilia of meglomania. What I want to say is basically the next step from that. The idea is to come straight out and tell her she's full of shit. As for it backfiring, well, as I've said, I'm pretty sure there's no avoiding a fight anyways (hell, pissing her off would probably help us; an angry opponent is more likely to make mistakes). It should also be said that what I'm planning to have Iku say is perfectly in-character.
As for how much I've been looking forward to it, well, that's just me; I get that way over all kinds of things.