Question about how healing works
If you give 5 sec of regen, and the other person then gives 2 seconds of regen, your regen will tick of its 5 seconds at your hp. And when it would expire, the other person’s kicks in for their 2 seconds worth based on their hp. The regen stacks in duration, meaning each new application queues behind the last one applied.
For your last question: From the wiki it states, “When applicable, the stats of the character that activated the finisher are used to calculate the combo effects.” http://wiki.guildwars2.com/wiki/Combo
I hope that helps, 
~EW
(edited by EphemeralWallaby.7643)