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After messing with this quest for hours, it seems the only way to successfully close this quest (while sparing Marg) is to talk to Grog *first*, agree to look into it and then tell Marg about Grog's offer (Choose that option FIRST). You then have to back out of the conversation (take your leave). Otherwise, if you go through any of the dialog regarding paying the "toll", etc, you will not be able to turn the quest in after killing Grog. Additionally, only the character who first talked to Grog/Marg seems to have the options necessary to further the quest.
Terrain transmutation using lava from the cloisterwood or deathfog from the coast allow one shotting Grog. A strategy I've used a few times to avoid trying to get close enough to a creature to transmute terrain without triggering conversation is to cast On the Edge on myself, then Shackles of Pain on Grog, initiating combat, Grog freaks out and attack but didn't have his armor up when I cast, and then I step into lava or deathfog. I might die as a result but this works on any character you can get their physical armor down on, Grog was the first I tried it on, I did it later on "a floating boss in an area with lots of lava which kept spreading" and at one point I did it to "an enormous oceanic type creature that kept spamming summons, by doing an overpower, shackles of pain, and smashing a deathfog barrel on myself."
I sit possible to pass Grog's speech check about the high bridge tolls?
With 5 persuasion and 25 int I failed Grog's speech check. Seems like you get forced into this quest.
I was able to pass Grog's speech check with persuasion of 6. Had a base of 5 and a medallion with +1.
use some air/eletric attacks to perma stun them. They got no magic armor. I did it just by swapping some runes.
If you kill Marg, you can go back to Grog and go through the same dialog you had with him before. Eventually the option to tell him marg is dead becomes available.
1) I couldn't report to Marg that Grog was dead, I think it's on a similar timer to merchant refreshes, just do some other quests a while and come back you should be able to report to Marg that Grog is dead. Probably similar if you kill Marg.

2) The rest the comments have a lot of creative ways to win. I just examined the trolls with high enough loremaster in the group to determine their weaknesses and resists. Whatever element they have negative resists on will remove their regen status effect "troll blood". Have a tank char setup, up front.

3) Grog has better loot than Marg btw.

3)*spoiler* If you kill Grog, Marg will "adopt some of his business practices". This forces you to either pay 15k gold to cross the bridge or fight him I could be wrong but I didn't see an option to just leave without crossing the bridge. A lesson about competition in there I suppose. Still made me sad to kill him though :( *spoiler*
I killed Grog for Marg but then Marg just attacked me. I couldn't even talk to him. wtf?
Use Living on the Edge and Forced Exchange. EASY.
If you talk to Marg first and tell him a story to pass the bridge then when you meet Grog later and ask you to kill Marg, you won't have any interaction with Marg anymore, only option is to kill him. So talk to Grog first or do not talk about crossing the bridge to Marg first