Headlight switch , is it the cause? or the curse?

For your fuel & temp gages, you should get a modern electronic "voltage limiter" ($25 ebay). Many posts here.

Sounds like you know how to trouble-shoot, so you will work thru it. On both my 64 & 65 A's, I removed the entire dash wiring and cluster and reworked them on the table to repair former hacks. The pins on the cluster's round connector often loosen and need resoldering to the PCB. I totally re-wired the engine bays, using a modern fuse/relay box (search post).

I suggest not bothering the guy who sold you the car. You must expect such issues when you buy a classic car. You will be happier in the long run to avoid quick hack fixes and work thru things methodically. Indeed, might be best to strip the whole dash innards and fix at once rather than pick at it over time.