Ok, if that's the case, then what I would do is find/use one long screw( +10mm) that could cover both links, plus the servo tray bracket, and still have enough length to be able to fit a nut.
If I understand you correctly, you seem to think that there are 2 separate front upper links. If so, then this is not the case. The front upper link is actually a single V-shaped link. So, only a single screw is needed.
I've been doing quite a bit of wrenching to my SCX24s and I have not come across this issue of not being able to back a screw out. TNT's suggestion is pretty clever, so hopefully it'll work for you.
For part B: the front upper link is easiest to access via removing the top 2 screws of the front...