(Note: I don't really support either side of the argument on whether they should appear or not. Sure, it would be cool if they did, but they could just add Hoenn islands instead....)Isn't the Sevii Islands exclusive to FR/LG? Why would a cluster of landmasses suddenly disappear from one region to reappear in another?
They wouldn't make it "disappear" from one region. The Sevii islands themselves could be considered a mini-region, it's not THAT close to Kanto. And besides, to access them in Kanto, you need to use a ferry anyway. So a ferry could also be used in remakes to go to them as well, IF they did appear in ORAS.
I don't think Red catching Mewtwo is canon. Zekrom & Reshiram are the only one who were really catched.
Correct. Also, assuming only one Mewtwo was created (which may be a bad assumption), it would be impossible for it to be caught and then also appear in Kalos.