Everything I say is just an educated guess.
1) Not that much faster. The C's 4 pre-season games last year lasted 2:07, 2:15, 2:05, 2:19. Compare that to 4 games at the beginning of April: 2:21, 2:01, 2:12, 2:25, not that different. (That's tip off to final buzzer, doesn't include intros, national anthem, etc.).
2) I would guess yes/no. Fan behavior (drunken, rowdy, disruptive fans), for a Sunday afternoon pre-season game is going to be as mild and kid friendly as ever, especially when the Patriots and Red Sox are playing at the same time (but as away games). Crowd noise though will also suffer because of it (though a 6 year old probably won't notice), but these types of games tend to have stuff for the kids just to get families in the door. (NOTE, I DON'T THINK I'VE EVER BEEN TO A PRE-SEASON GAME).
3) I'm a big fan of waiting for tickets, there's definitely last minute bargains to be had. Pre-season + Patriots and Sox playing at the same time (though not in Mass). This is not a high demand ticket. At the least, you won't be penalized for waiting.
Just to emphasize, I don't think I've ever been to a C's preason game, but I've been to a lot of games in a lot of different cities for every major sport. So just my educated guesses, take it all with a grain of salt.