Bruh ... I just fixed this shit in my 95 last weekend. Didn't take any pics tho.
Same issue it seems. When I drove the car home initially I though the back rest adjustment was stuck and would not go more upright. My passenger side also stopped at about the same spot, however it seemed more comfortable/upright. So I checked it closer over the weekend, took the seat out. You need to separate the plastic seam the bottom of the seat cover, one more plastic clip holds the cushion on the lower back side. Remove the head rest and headrest trim, just slides up/out for both. Remove belt holder. Slide cushion/fabric cover up slowly side-to-side and remove.
You'll find the backrest frame will be cracked and some of the spotwelds will be broken. I took a ratchet strap to the top corner of the backrest to the base and bent the backrest back to position. Then did some repairs to the spot welds with several rivets, and repaired the cracks with some small plates/rivets. Put cover back in the same fashion as removing it. It worked great, no more lean.
This is on the V6/GTS seats like in your 1st pic. They are not the most comfortable to begin with, but this fix is what a lot of Mustang seats suffer.