I can't speak for the homosexuals, but most of your questions are a bit silly from even a heterosexual perspective.

"Who takes who's last name?"

My wife didn't take my last name or vice versa. She's still "Alvarez", I'm still "Yocom".

"Who is walked down the isle?"

We also got married in a spartan ceremony in the Salt Lake County Clerk's Office: Assistant County Clerk to officiate, two of my friends as witnesses, no guests. No aisles, and certainly nobody getting walked down them.

"Do both guys attend the batchlore party?
Do both women attend the batchlorette party?"

We also didn't have bachelor/bachelorette parties. I would assume gays might have two parties, just like many het. couples have separate bachelor and bachelorette parties.

"Do we call the married couple Mr and Mr? Mrs and Mrs?"

Yes