Can I cancel out the Apprentice stone debuff by having an atronach with the same element as whatever caster I'm fighting?

You can't truly cancel it out that way, but if you can get the caster to focus their attacks on the atronach then the weakness to magic becomes a non-issue.

What's the magnitude for quick reflexes, supernatural reflexes, and both ranks of steady hand?

bump

I think Supernatural Reflexes is like Slow Time (1 word), that means it slows time to 30% of it's normal speed. However, it also slows you down a bit so the actual speed might differ.
Quick Reflexes doesn't seem to have the same effect as slow time on you. I'd say it's as strong as Slow Time (2-3 words), 20% / 10% of it's normal speed, but with a lower impact on yourself.
There was a page in the uesp that stated that steady hand is bugged and it actually slows time by 50%/75% instead of 25%/50%, respectively. I believe this but I might be proven wrong (the uesp says it fixes this bug).

So, I have to do the Quest Proving Honor to get part of the Fire Breath Shout( stupid quest-locked Walls) and I just got Serana from the Crypt. If I don't return her home before I take the quest, what'll happen when Farkus takes the follower spot?

Wouldn't they just both follow you since they're quest-based followers?

That's what I think, but I wanted to be sure before I tried it. 

Just save and try it. You can always come back.

Went through no problem and now I have a bunch of silver weapons that are vastly more powerful than what I had.