If concerned I'd sleep top down. If the roof collapses on your face in the night who's gonna help you. Would it ? prob not. we've had about 8" overnight, unexpected. But why risk it ? Peace of mind helps me sleep.
And it seems to me it would be a lot easier to clear off the roof in the morning if it was down.
Tarp is a good idea but snags on our rack.
I've slithered over the FWC roof but haven't walked on. Has anyone ?
Have also used wood blocking for backup (1x6s) but found them hard to stay in place. Now sometimes use quick claps, reassembled with the handle and stop flipped outwards. Under tension they stay in place. Use them when carrying a canoe for eg. Don't know how strong but reckon if placed in the overcab area the rear roof would go down first.