Maybe your arse is just too fat, I always find if I am doing more exercise running etc to muscle up that long trips like I do are easier to cope with..... a lot of numbness is down to circulation yes even in the nether regions so try that for a while even some squats 3 or 4 times a day running up and down stairs a few times two or three times a day would help with the glutious maximus and the circulation lol
as my old pti used scream at the troop run fat boys run pays to be a winner lol
As for the pillion, Rusty if you suggest the above to her your a braver man than me pmsl