I love Rebecca - spelling it the classic way. It definitely fits with all the other siblings, and is the most 'pleasing' to my eye (of course, that's just me lol) I've always had to repeat my name and spell it out to people, so the classic spelling is much more likely to avoid that issue later in life.
My thought on shortening names, is to put the 'full' or traditional name on the birth cert, and then use whatever name you like for the child - in this case, if you know you're more fond of a 'pet' version of Rebecca, then just pop 'Rebecca' on the birth certificate and then call her Beccie, Becca, Bec etc from the get-go. That way, when she grows up she at least has the option of using the 'grown-up' version as it *is* her name, kwim? Like for job applications etc, she can put 'Rebecca Smith' on her resume instead of 'Becky Smith', as potential employers are probably more likely to warm to a classic name than a 'nickname'
Good luck! FWIW I think the Dutch spelling of Pieter to honour your uncle is lovely - poo to DH!