Good luck with this, Blodwyn!! We had a Thorn Discovery a few years ago, and had good trips on it in Scotland and Finland, but we ended up preferring solo-ing, and it went to friends on 'loan' - they're still using it!! I think my wife felt I was too fast downhill - I probably was.
Re transport on car . . . on our campervan holidays, we took the tandem on the back of our VW van. Wheels off, fork turned round, and I made a cradle for the bare-as-possible tandem frame to sit on our bike rack. Cradle was made of wood, secured to the bike rack, with two boat rowlocks on top of the cradle to support the frame. This worked well, the bike rack we had was a Thule fold down sort - i.e. fixed to the van tailgate, the bike support folded up / down when not in use/ in use. Everything secured by straps etc. Thinking about it, quite why I didn't just use the folded-down platform as a base to lay the frame down on flat, I don't know - perhaps my tendency to find solutions that are more complicated than they need to be!! Or perhaps there was too much overhang.
If I find the cradle in the dark corners of my garage, I will post a photo, but I have a feeling it's gone.
An alternative might be the sort of rack that is two prongs out of the back, secured onto the tailgate, might work better, but that type could not be fitted to the van.
The length of the stripped -down frame was just within the width of the van if I remember rightly, and I guess a XC70 is also a fairly wide vehicle.
I have seen tandems on roof carriers, but as the carriers usually only have one 'frame support' upright, I would be concerned about them flexing a lot - never tried that.
Or would the frame fit in the back of the Volvo?? Leave the rest of your kit at home?!
Good luck anyway.