Everything else you said convinced me, but the Fort is the first place to be hit by the securitrons. As soon as you power them up you see a massive explosion over at the Fort, which I took to be them rushing out of the vault and obliterating the Legion.

My question is how does Caesar survive a firebomb on The Fort?

Dunno they're all officially canon, aren't they? NCR & the Legion surely is. I don't care to be honest what you follow, it's called choice. As for future Fallouts maybe it would be refreshing to scrap all cannon based factions - regurgitating BoS, the Enclave and NCR gets a bit boring. Maybe something that evolved from their descendants or so but with a different name and outlook on things.

Was a long time since I did that quest so it's possible I don't remember it.

When it comes to fracturing I think the NCR would most likely fall apart long before the Legion. The NCR is made up of several different states that all want some degree of independence and power, and many people don't agree with the way Kimball is running the country at the moment. The Legion on the other hand has a much more monolithic government, Caesar himself stated that one of his long term goals is to create a government that will not fall apart. The fact that people are fanatically loyal to a position of power (Caesar) instead of any specific person of region means that there will always be order as long as there is a chain of command. The only source that he have that the Legion will fall apart without Caesar is Marcus's comment, but he's never even come in contact with the Legion and he probably knows very little about them. Even if every member of the Legion's leadership dies at the second Battle of Hoover Dam, the position of Caesar would probably pass on to whoever was left in control back at Flagstaff.

As for the ending, I'm not too sure. If the NCR or Legion win, it will open up the possibility of a much larger war between them in the future. With the Yes Man or Mr. House ending however the war might be postponed and the series could focus on something else for a while. Either way there are a lot of options.