Who did you pick? And why?

No, if you have friendship, he will side with you no matter what according to the wiki.

Yes, at ~80% friendsip he will leave you, but later you can convince him easily, and on 100% he will not question your choice. This is not correct on wiki?

Huh, than maybe I didn't have enough friendship with him though... 

Yes, I'm glad, that Fenris not as tragical on the Mage side, as Anders on the Templar side.

This is just an approximation. I dont know the exact value.

I don't think, Hawke can convince him, if s/he did not care about him enough (friendship/rivalry don't matter, yes). It's would be not logical. (But I don't know, never happened.) 
(Not diplomatic, sarcastic or direct answer here, this is a "neutral" option)

UPDATE: I found this, you can see, fenris can resist to Hawke's ask (I still dont know, exactly how much firendship/rivalry rate need for the positive answer, but it's important, I suppose):
(I nevers saw before, but...Anders is great – as also the others!)

In my first game i choose Anders because i really liked him in Awakening. I`ve played the other DA2 romances and he is still my favorite.

My first Hawke, a sassy male mage, flirted with everyone but ended up with no one.
It kind of... just happened. Every time I apparently was supposed to pick the romance dialogue to lock in the romance, I ended up skipping it since the dialogue just sounded so cheesy and wrong coming from my character. Or then there was an option which fit better to the way I was roleplaying him. I do remember Merrill actually confronting him about it in his house but the way she went on about it just made it kind of uncomfortable and he backed off.

My second Hawke, aggressive warrior, ended up with Anders. This Hawke was fiercely protective of Bethany, hated the Chantry and Templars, even more so when Bethany was dragged into the Circle. Anders' passion against the system supported his thoughts perfectly and gave him hope that one day Bethany would be free again. He was with Anders to the end.

I guess I just go with what feels right for my character. I don't really plan ahead.

When I was at the Gallows on my Fenris romance play through, Isabella's "Goodbye" Scene was very touching. This has given me more appreciation for her, and will be a good bouncing off board for when I do her romance. 

You gotta dig deeper for the gold, but it's there. ^^

I still haven't seen the best of Merril. But what might have been hanging me up is her appearance being very alien child, and so I've added a mod that's matured her.  I know, shallow. Elves in DA2 loook off anyway, and then Merril is especially young looking with her face and clothing.

numphet you should try it! My canon F!Hawke rivalmanced Isabela making her selfless and better person, it was worth it I love Isabela-like characters, and I love to soften their hearts even more!

Now I'm doing rivalmance with Anders with a "pro-mage"-neutral warrior M!Hawke just for fun, it's ironic that my canon gay Warden romanced Alistair and now that I have the posibility to do it without mods my canon Hawke is a female XD I love females as the main character in fiction, but with this game who let you to customize your character it's difficult to create a female character for me who I identify with.

I think I will do all secondary and side-quests before starting any romance in Act 2, to try Fenris with friendship. Planning to do the opposite with a pro-mage M!Hawke with a rivalmanced Fenris and friendshipped Anders in the future.

I think, the rivalmance is creepy with Anders. This will slowly kill him. This fact is creepy enough, as I see. I like Anders friendmance, I like, that he want to move in Hawke's home. But if you like kill him, the rivalmance is perfect for you.
Yes, Fenris rivalry is very good, and more reliable with (pro-)mage, but the friendship also, good.