Didier, thanks a lot for the pointers, but in this case i believe to be pretty much experienced with (jwm; i mean ...

)
I could pass you the setup i was used to use with debian non-systemd in the past - with jwm as standalone DE. And i also know pretty well the manjaro jwm openrc flavour too. I learned from them to have a very small $HOME/.jwmrc which consists only of some lines (like this:
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Include>$HOME/.jwm/groups</Include>
which point to $HOME/.jwm/groups. The same goes for several other settings like start, theme, menu ecc. ecc.
To integrate jwm into Mate DE, i'd not need the menu option, since Mate provides it. Same goes for the start configuration ...
The problem is:
With any setting and configuration, the decoration (i.e. the frame/window title) of the window does not work, the jwm drawn windows are all placed *OVER* the mate panel and the do not have the minimize/full/exit buttons. I do not see, where the error might be (if at all). Rather, i'm afraid , it's not really "love" between Mate and JWM (?)