It sounds like the unit needs to go back to Renault dealer to be fitted with a proper seatbelt as you need to be able to move in the belt, eg: blindside reverses. Extenders could cause their own problems especially as most will be Chinese tat with faked BS markings or faked brand names.
As for 5th wheel, can’t you move it? Pretty sure the POS Renault we’ve currently got has a lever for moving it and all other trucks I’ve driven definitely have one. They likely set it ultra close by default as its “efficient”, not thinking of the driver.
Split coupling - alright unless it doesn’t go into the 5th wheel jaws properly and you move forward to reset, forgetting the lines are on. Or of course the day you forget to put the trailer brake on. If you’ve never done it, you will soon.
Btw, best idea - if its a Renault, just tell them no thanks and save yourself hours of frustration every day. I’d rather drove a 20 year old Iveco than a Renault.