After all, there are geneological books around that state quite categorically that Darcy is exactly as you say- a norman name, and it was with this in mind that I was killing time in France by reading phone books and driving around the countryside with tatty maps in hand.(well with a newborn and a hyper toddler in tow you really cannot just drink the time away, can you? ;-) ) It was only when I couldn't find any trace of an Arcy that I started to look elsewhere. It still might be there, of course. I might have missed it whilst looking at spring flowers, or changing children's underwear, or persuading the local gendarme that my international driver's licence was in German for a reason, you never know. I suppose that there could be two versions of the name, too- a Norman one and a Celtic one.
As to the apostrophe, well, I suppose human nature being what it is, and because in the bad old days even princes were illiterate, people got to spell their names any way that they wanted to, and poshed them up as they saw fit. One has only to think of names like Smythe, and hyphenated-Jones to realise that there's more than one way to fake your ancestry. After all, Shakespeare couldn't decide how to spell his own name, could he?
I just have this feeling, and I've had it for a long time, that Jane was on an irish kick with P&P. If so, it goes way back before Tom Lefroy, back to when she was about fifteen, or even younger, when she made herself a fake wedding certificate, and married herself off to a Fitzwilliam. The irish connection turns up in Persauasion, as you say, and also in Emma,/i>, with the Dixons going off and abandoning poor Jane Fairfax.I guess we'll never know for sure, will we?