1- RivNut type threaded inserts are a really poor way to create a mounting boss in a thin wall tube, except for the other ways As the insert is compressed during install it's threads can become deformed and if not followed up with a thread chasing can allow bolts to very easily be started improperly. If the insert in in a tube that has a large enough opening at either end (think of a seat tube) then the insert can be removed from the tube wall and also from the tube's interior. The insert's lip (on the outside surface of the tube) can be pried up so that the insert "falls" into the tube. If down carefully a fresh insert (of the correct spec) can be installed. I've done this quite a few times.
2- Sorry I don't have that memorized
3- Casing end caps with a snout do work well to reduce the inner cable's friction at the cap's end. Until the snout breaks off or wears through. Then simply install another.
4- I would not ride a bike with critical fittings only tightened to half the needed amount. Do the job right the first time even if you need a third party to help out.
5- Since I don't know what design, brand of brakes you have or how the wheel/dropout securement is done I have no base to advise you from. But we deal with this stuff weekly at work and generally can get longer lasting results then you are experiencing. One question I would ask is whether the wheel is moving within the frame/fork, whether the caliper is moving about on it's mount (assuming disks).
6- Without seeing the problem in person I can't say much. It sounds like the fork steerer has a compression plug in it and either the plug is spinning while you try to tighten it's draw up bolt. But the plug's expanding cone could be damaged, the bolt could be damaged of some other thing is going on that alludes my long distance assessment skills. Andy