Well I guess you can't get in there enough to "rebuild" the captive nature of the nut so....Get bolt started in that nut, adjust door and tighten other two bolts, then reach in and hold middle nut and tighten middle bolt. Since it is a square nut, you can make a long, thin "wrench" from a 3/16-1/4 thk metal strap with an opening in the end. Drill a hole same size as flats on nut and slot to the hole to make a "wrench". This piece can be bent to accommodate access angles... Or hold it any way you want. Be sure to oil and chase those other two threads (nuts), very gently, so that you don't bust them loose when tightening bolts.
There is another method I've used, piece of 1/4-3/8 thick bar, maybe 3/4"-1" wide, maybe 2"-3" long. Drill and tap, near one end, for that bolt. It will spin and catch on a place on firewall/fender/wherever...if there is a place. It will float until final tightening and then be "captive". Just have to get it started on bolt by reach in.
True "Captive" means no wrench is necessary and/or it won't fall off when fastener is unthreaded. There are captive floating, like yours used to be and just captive, like a weld nut or press nut, etc.
There are captive and captive floating screws also.
