There is an interlock that prevents folding the backrest forward until two things have happened:
1 - the seat bottom cushion is raised out of the sitting position.
2 - the associated door is open.

I suspect that something has caused the seat back interlock to break. Probably the interlock associated with condition 1. This may be what is preventing you from putting the seat back upright.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech