Character looks pretty solid to me, though I've never seen the show.
The additional explanation on the Powers helps, but there's still some vagueness there. For example, what exactly does William get for his 4 Refresh of Mimic Abilities? What abilities can he mimic, and what limitations are there on his mimicry?
Primarily skills, presumably also stunts. In the original [-X] Mimic Abilities, you can "steal" X different skills or stunts and use them as your own. You can do this to theoretically anyone.
For my sensate write-up, I treated it the same way except
a) you can't take skills/stunts from anyone you meet like the original ability but
b) you can only draw on abilities from within your cluster- how much you can borrow at one time varies on the experience level. Initially, they draw on random abilities instinctively/by accident, but by later in the show they theoretically can draw on each others' skills more (another clustermate's primary skillset, instead of just a single skill). They can swap these at will.
I haven't watched the show either, but I think the Mimic Abilities is more for stunts and skills than powers.
Correct. The only powers in this world are that of sensates, whose don't possess any supernatural strength/speed/recovery etc. Just the connection to share their human skills/stunts.