1-You can thin oil paints with some mineral spirits, as with any other medium, it can be problematic if used in excess.
2-I have only tested regular liquin, it didn't yellow at all.
3-Cobalt is a strong drier and using too much can be bad for the paint film, I have never tested it but it is well known that it can increase darkening/yellowing, etc. Here is a quote from AMIEN:
Cobalt driers are oxidation catalysts, drying the film from the top down -- this is not the way drying oils normally dry. Excessive amounts of drier can cause the paint films to wrinkle and yellow. Moreover, the cobalt drier itself can be unstable once exposed to air in the jar.
We therefore do not recommend the use of these kinds of accelerating driers in oil paints.
Searches here will turn up many threads discussing these topics at length.