I would recommend playing from top to bottom. So finish Leon's quest, then Chris, Jake, and Ada's. Regardless of any order you may choose, I would strongly advise to play Ada's last, since her campaign intertwines with all of the others in a fashion that may spoil some of the story, or just leave you with a lot of questions.
I finished my Vet playthrough the other day and am going for Professional. Take a closer look at each of the characters. Playing as Leon or Helena will net you slightly different weapon's and play styles, especially in Jake/Sherry's campaign.
The button mashing will subside. It seems like no matter what once a zombie tackles you, no amount of mashing will save you from death. But again, this goes away near the end of Leon's campaign, and is fairly non existent in the rest of them (but there is some at times)
That's fine. Just play Leon's campaign after you finished Chris. For instance you're playing Chris and Pier's campaign. If you choose Pier's over Chris, you will get a different weapon, and some slightly different areas. What i mean by different area's is, just like RE5, if the players get separated, you will play his/her area, until you are reunited. So for those slight detours in game will be different depending on who you select. To me, the real benefit of different players is the different weapon you get. Not a drastic difference, but enough to keep things interesting.
The game doesn't so much focus on one area, but the stories all intertwine, which is very clever might i add. The story is what makes this game great in my opinion.