For #1), my problem was corroded copper traces in the flexible board material on the back of the gauge cluster. I know how to solder, so I gingerly removed the flex board, cleaned up all of the corrosion, and soldered in some 18 gauge wire to replace the corroded traces. It's like using a jumper wire for an open circuit. Then I used hot glue to hold down the wire right over the old trace. Spray some electronics sealant over the whole thing, and put it back onto the gauge cluster. You should check each segment of copper trace with an ohmmeter or continuity tester. Sometimes you can't see the cracks.
For #5), if cleaning the contacts doesn't work, look for damage caused by corrosion.