It's going to be a suck it and see exercise, as it were. The slimline Recaro SL3510 seats are very uncomfortable for some people, it is not at all forgiving, but its firm base may be quite comfortable for someone with a bad back. If you sometimes find it relaxing to lie on the floor during bad back phases, then the seat may actually work for you. Until you try it for yourself, you won't know. As you rightly noted, the seats also have fractionally more leg room, which may help. Being at the back may help you get the spare seat, otherwise perhaps consider a POUG, UuA or AUP for cabin crew administered medicaments.

Note also the risk of equipment change is very high at the moment.