sorry, i dont have a pics. Somehow I can't find any. But I can guide you if you want to fix it.
you have to remove chair first, and roll fabric away from the hinge.
you will see 2 metal rhodes going into the hinge. The problem is the one which is going into plastic fitting.

what happened in my case, the hinge was rusted, and rode rotated inside plastic holder, so when i tried to latch the back vertically the hinge did not allowed it.

you need to unscrew the hinge and see condition of plastic insert which holds metal rode. It is funny that quick release rode is not using plastic holder and hole looks like alen wrench. But reclaining rode is rectangular shape which is sticked into plastic holder. Be careful when removing plastic holder, not damage it.
so when you figureout how hinge is working you will understand the problem immediately.
if you need more advise send me your pictures and i will explain you more

 

A.. GOOD JOB !! And thanks for updating the forum on your repair and success.

 

if passenger seat doesn't latch vertically then you have to change the cushion over seat. you can try with best air mattress pump . your seat would be more comfortable with that. you can sleep over mattress. your back pain will recover soon. regular seat is not so healthy that's why you need to change your mattress immediate.