Each Spidey having a different voice actor - Ultimate's a teenager, Noir needed to sound grittier, and 2099 isn't even Peter Parker.
The order of the Spider-Men on the cover is by alphabetical order by dimension. 2099, Amazing, Noir, Ultimate.
The fact that Amazing Spidey could defeat the Juggernaut in a fist fight seems impossible until it's revealed that when activated, the Tablet interferes with the power of the Crimson Gem of Cyttorak, so it may be safe to assume that it's been doing it the whole time, even when it wasn't activated.
Under Mood Whiplash, it's noted that Ultimate Spidey continuing to make one liners during the Carnage level felt out of place considering what's going on around him. But think about it - He's still making one liners so that he keeps his head & doesn't freak out from Carnage killing everyone.
During the Deadpool level, Ultimate Spidey is asked where the game is in continuity with the comics - There's a 6 month gap between Ultimatum/the end of the first volume of Ultimate Spider-Man and the start of the second volume, so the game could easily fit in there. Incidentally, the game was released shortly after the beginning of the second volume.
If Noir dies from Vulture biting him in the punch-out segment, it's likely that it's because his face has been bitten off.
If you take Carnage's stage into consideration, if Ultimate Spidey failed, the damage would likely have been long term and nearly impossible to fix, if it would even be fixable in the first place, and even then, the end credits become a lot less comforting and humorous when you think about this more and more, because it confirms that there are still a few zombies left.
Judging by Deadpool's comments about Spider-Woman, Ultimate Spider-Man's segments are set after the Clone Saga, meaning that you've left Gwen Stacy trapped under burning rubble.
Not likely - By the time the game was released, Gwen had been separated from Carnage for some time.