a) The standard seats in the C70 are the best I've ever come across. I regularly drive a 8.5hr, 550 mile journey with no pains or aches whatsoever. In fact, if I didn't have to stop for fuel after 400 miles, I wouldn't stop at all. I had to have a back operation a couple of years ago and if I didn't have Volvo seats I honestly don't think I would even contemplate the journey.
b) No bum-warmers (although depending on where you live that might not be that significant). It certainly makes winter mornings in the UK a bit less miserable.
Reupholstery depends on the shop. You'd almost have to price around a few shops in your area. The cool thing though is that some of the better shops can do cool things like make your bolsters larger and use other fabrics like an Alcantara (synthetic suede).