Only know #2 and not completely sure, but I'm fairly certain:

you need a higher crit score the higher level you are. You know this because with every level your crit decreases and you gotta put new points in ferocity to compensate.

So I assume that in order to reach higher numbers, you need good gear to scale it further.

is there a hard cap? I don't think so, based on this observation.

is there a diminishing return/soft cap? Possibly. But again, I don't think so.
playing a full Agility build, putting 10 points into ferocity would only net me about 1% crit, whereas if I put 10 points into it at a lower level, Id get a whole lot more.

So it seems to me that it scales down solely on levels, and so you need a higher crit score to reach same numbers. But same goes for other stats (ailment chances, attack speed etc) - each individual point simply loses value the more you have at your disposal, so you're not really losing anything by putting them all into crit.

could be wrong but based on my observations this seems accurate.

Hope that helped a bit :)

p.s. For #1 I would simply assume the stat does nothing for you, But I can't say for sure.

yeah that does clear it up alot thanks will probs need to reset my characters attributes now that I understand it more :D

Did u find answers to your questions ?
I want to make a build around stasis/curse too :p

I just realized as well with a test that the attack critical chance decreases with level.
The attack critical chance score does not, but the percentage does.
Looks weird to me.
Is there any official explanation about this??