Omg, so many things to answer... I don't even know where to start... lol
First, about people saying that "Arakawa is not implying anything", there're quite a few people out there who, first: think Tombow's translation of the comment is wrong, second, that there comment has no implications whatsoever and, therefore, Royai is not cannon. In fact, I've been discussing that with a dude at the FMA wikia for like 3 days, now... exausting, I might add. ¬¬'
About cannoness, the word "cannon" comes from Catholic church, meaning a text is "original", which means it takes part on the "official texts" of the Bible (which, according to them, are all inspired by the Holy Ghost, when they were written).So, basically, cannon only means something that is "intentially put by the author"
, as opposed to something a fan has created. So, "being cannon" does not mean being "romantically involved" or anything of the sort: it only means the author understands that "couple" as being "in love", whether they might act on it, or even be AWARE of it.
For example: Sakura and Syoran from CCS are a cannon couple not since Syoran started to notice his feelings towards Sakura, but from the day one, when CLAMP decided that their relationship would be of a romantic nature and, therefore, be a contributing factor to the character's decisions throughtout the development of the series. And even when the feeling isn't reciprocal, it IS cannon, meaning, the cannon is that "someone is in love with someone else and that other person doesn't love him/her back". For example, Faye Valentine is in love with Spike, but Spike loves with Julia. So, THAT definition is also cannon. Is the couple Faye and Spike cannon? No, but the fact that Faye is in love with him and he's not, is. (let's not enter the fact that Spike might have feelings back, cause that would be complicated)
And "cannon" doesn't necessarily refers to couples and romantic involvement, actually, but of ANYTHING and EVENRYTHING in a work that can be considered to be "the author's mind".
This is why cannon is, by definition, something rather subjective, because it depends on the intentionallity of ones writing, which is something that it, sometimes, not clear even to the person who's writing it. (you think that Saint John KNEW he was being 'inspired by the Holy Ghost" as he wrote the Apocalipse and that the words he wrote weren't 'his' in the first place? He might suspect it, but certainly not aware of it.)
That being said, I understand that the FMA fans who insist on saying Royai is not cannon are either unaware of the true meaning of Cannon and, therefore, believe that a stablished relationship or even the awareness of 'being in love' is required to consider a couple to be cannon, or even believe that, in order for a couple to be cannon, they have to be "exclusive", so, therefore "Sakura and Sasuke being cannon elliminates the possibility of Sakura and Naruto being cannon". Or that they have established in their minds some other understanding of the works by Arakawa and won't change their interpretations unless a blunt statement by the author contradicts such an interpretation. Which is something someone can pass through with many a mangakas, but means a "Death sentence" for the works by Arakawa, since she's the sort of author who expects US to be "clever enough to get it, even without me saying it".
So, is there any chance that WE are "over implying" towards thinking this comment by Arakawa proves that Royai is cannon? There's always a chance, since she didn't say the "words itself". But, I trully doubt it and would be taken aback if she was to say in the future that "it's not". (and I'm sooo expecting that Arakawa would put something blunt in the FMA Chronicles or make that Gaiden she said she was thinking of doing... it would be so fun to see these "Saint Thomas"'s go
But, anyways.... About the marriage itself, I guess people already said everything there's to say about it: Riza DID mention that she would protect Roy "until he reaches his goal" and that makes total sense, because, it was their promise to do so, after all. And I really do believe Riza would leave the Military after Roy becomes Führer, since she's there almost solely for Roy and that she doesn't really like
it. She does it (like a Mulan sort of thing) for her sense of duty, not for taste. Moreover, noone ever said the First Lady of Amestris has to be a "house wife" and she can very well be his "civilian assistant" as Führer, since the Fraternization regulation is something specific of the Military personnel, because of the obedience oath and personal privileges, both of which wouldn't be an issue in case Riza worked as a civilian...
But, I really don't know... THAT we would really have to ask Arakawa. XD