I just saw this with my mother and we both liked it. :)
I'm coming at this as someone who has not read the comics. Most of the reviews I've seen have concentrated with how much has deviated from there. For me, I just went to watch it as part of the film franchise. I'm suspecting that the vast majority of casual viewers are probably coming at it from that angle, too.
(1) Childhood years:
From how you describe it, in film-making terms, this wouldn't have really been possible. :) Or, if it was, it would have been something changed in earlier drafts. the studio and audience would be paying to see the guy in a recognised, adult form. They could've had him refer to stuff, I suppose, but by going on what you allude to, that would have meant an awful lot of exposition and left the viewer wondering why they didn't just make the film purely revolving around that.
As a story in its own and unique right, the development which we saw worked for me. It got the character from A to B in a plausible way and showed emotional development.
(2) Haven't seen 'Watchmen' and don't really intend to. I liked the montage. Not sure what you mean by every war ever known. :) Canadians did fight in the First and Second World War. The first part was either the Civil War or War Of Independence: The guy could have potentially been involved in those, because we were shown he and his brother went on the run and never looked back.
(3) I think they should have brought back the original Sabretooth and Stryker actors, but I've got no idea what the pay situation would have been for their contracts. Stryker's probably wouldn't have worked, because this looked like it was set a couple of decades before or something. I agree that he needed some added ruthlessness and mental contemplation, but it didn't destroy the story or anything.
(4) I don't have a clue who Emma Frost is and most viewers probably won't, either. The mutant could have been a generic anyone with that power. It's not really important. Cyclops' inclusion makes for an intriguing retrospective dynamic, when you watch the later films. No, it's not necessary, but neither were the Kitty Pryde cameos in the original films (or Beast's in the second!). :)
(5) Most of your problems with the character realisations seem to come from them deviating too much from the comic interpretations. For me, they were just film characters and, in their designated roles, they were fine. I liked that the teleporting and boxer guy had a past fighting with Wolverine. It gave the teleporting guy a reason to join up with him, later on. I don't remember 'Blob's' weight being described as an eating disorder; I seem to recall 'Wraith' saying he just sort of let things go, but I could be mistaken. This is actually fairly common for ex-military guys. :)
(6) Bradley was shown having a purpose: He could manipulate anything which used electricity. Remember how he forced the elevator to over-ride the stop? The later development showed how he was just applying the talent to live a quiet existence and forget about stuff.
(7) I didn't understand why Gambit interrupted Wolverine from almost killing Sabretooth, but he came across as a neat enough character. Not awesome or fantastic, but he didn't need to be. It was obviously a character inclusion done for fans, but if a cahracter doesn't need to do any more than a scene demands of them, then they don't have to.
(8) The experimentation on Wolverine was up in Canada! It got moved out when Wolverine escaped, remember? :) It was already shown as abandoned in 'X-Men 2' (only partially reactivated, later on), so, that was fine.
(9) Deadpool's inclusion of new abilities was probably to make him seem cooler and give weight to Stryker's plan of putting many mutant abilities in one body. To me, that made sense. Again, this is something which might well have deviated from the comics, but in the context of the film, made sense and made him more formidable.