Wow. Quite a list.

My '86 245 had a wild temp gauge till I did the temperature compensator board bypass procedure (details at end of this post). Definitely do that first. It comes up on this board regularly. I'd guess that 98 % of the time, this is the cure for temp gauge errors of almost any kind (too low, too high, or needle position changes not related to engine temperature).

Fuel gauge problems
Lucky you, yours works "most of the time". In any case, don't follow the advice of people who tell you to hit the dashboard above the gauge. Possibly the previous owner did that, causing all the other troubles with your cluster. When not working properly, the cure is usually either to replace the gauge, or the level sender in fuel tank. Fortunately, that PITA job is a bit easier in a wagon than in the sedan.

I'd look for a bad ground somewhere also. This could be causing tach to fail when lights are on, as cluster illumination may be grounding through tach or elsewhere, fouling things up. The Haynes manual has a pretty decent schematic of the cluster. I haven't seen the Bentley, it may have even better.

You also could get a replacement cluster from a junker. Sorry, I can't tell you which years match. If you do, count on doing the temp compensator bypass on the "new" cluster anyway. Before you put it in - save yourself some trouble!

For the temp compensator bypass, go to the ipd site, find their bypass kit under "Cooling System". Have a look at the instructions (.pdf file) there, they are quite helpful. I did the bypass with that .pdf and a 2-inch piece of 16 gauge wire. Cut the wire off clean on the ends, no exposed length. Poke a sharp point up each end of the wire, that makes a narrow hollow that you can slip over the two pin connections. That way, no soldering is needed. Reassembled instrument cluster holds the wire in place! When you see where the pins are, you'll be glad to not need soldering. Maybe replace your thermostat afterwards, now that you can read the real temperature!