RE: the shift lever with the stuck cable, IF as mentioned above, the cable remnant has “bonded” to the shift lever by the action of corrosion, attack that problem first. For the rust on the cable, get some EvapoRust (available at auto parts stores or large hardware stores) and soak the lower portion of the lever in the solution for several days. I’m not so sure that I’d have the plastic ends of the lever in the EvapoRust tho since I’ve noticed a bit of “softening” of some plastics that have been totally submerged. That process should remove any iron oxide that might be contributing to the “bonding” and weaken the grip of the remaining corrosion.
Since, also as mentioned, the cable head on French cables is smaller in diameter than most cables in current use, a previous owner may have “smashed” the cable head into the smaller than now standard hole in the lever. Tapping it from behind should pop it out. Good luck.