Elial Brook and Deepwater Knight faction.
 

Starting from indifferent, how many times would I have to kill Elial Brook in Tox to max my Deepwater Knight faction, or at least get it high enough to qualify for the final turnin for Fiery Avenger?

451 times

You have to kill him a lot. Start with 3 hours or so before bothering to check faction. Also I recommend killing Aglthin Dinsmore to maximize your time spent in beautiful Tox Forest.

http://wiki.project1999.org/Aglthin_Dasmore

And how high do I have to be to be able to do the Odus Pearls quest in Erudin? I'm Amiable to the Deepwater Knights right now but the guy who gives the quest still won't give me the time of day.

Hmm, I dunno. As an erudite, I didn't have to do any faction work for that quest. I would recommend it for a level 11 anyway - there are many sharks in the way of those pearls.

I'm able to do it now it looks like. I had to con Warmly before this guy would even talk to me.

Elial is still giving me faction though and I'm well into Warmly. How high does my faction with Deepwater Knights actually have to be to qualify for the Fiery Avenger final turn-in (meaning he won't eat the turn-in and give me nothing)?

just max it

"Your faction standing with DeepwaterKnights could not possibly get any better."

YAY!