Threadhopping: Go ahead! The more the merrier. If I post a log I always leave it open unless it specifies otherwise, but you can totally jump in if you think your character would. Make sure to get the other mun's permission too!
Back-tagging: Of course! Life's too hectic for people to be tagging at mach 5. So I have no problems with this.
Kissing: Woman can kiss him, he'll be surprised if it happens out of no where, but probably grin and say something teasing and in an almost flirting manner. Can you tell if he's sincere or not? Men might get decked for it, but if they want to give it a go, go ahead.
Intimacy: See 'Kissing'
Attacking: Sure go ahead, but he can hold his own.
Injuring: Sure! he'll injure back if it's an opponent he can handle.
Killing: You'll have to talk to me either in PM, AIM, Plurk, what have you -- to discuss how this would go about.