1. Prob needs Rear Cylinder(s) and shoes .. and a bleed.
2. Will just be the Rear Earth point or the contact strips in the connector (when the rear light do weird things, it's because the earth is failing, so it finds a a way through somewhere else. That is usually one or two other bulbs, so the come on dimly)
3. Maybe wrong offset, or too wide tyres - 205 is the max without further mods.
4. Sills? Best to cut off the whole lot, incl the flanges and do properly. Not an easy, quick job, but if you are keeping it .. well worth doing! I always cut out ALL the old stuff .. never plate over

Structural and mechanical first, mate .. worry about the actual looks later