There are purposefully adjustable voltage regulators and ones that are adjustable but only to correct manufacturers errors.
This is an example of a non adjustable electro mechanical VR. Note that it looks like the one you have now and it has the wire would resisters underneath it.
View attachment 1716332272
This is an example of a non adjustable Electronic version of the one you have. Note there are no wire wound resisters underneath. It also looks like the one you have now.
View attachment 1716332273
This is also a non adjustable electronic version of your VR, it does not look like yours but will work just fine.
View attachment 1716332281
Adjustable versions exist and have a place to put in a screwdriver to turn a screw that will change the voltage set point.
You do not need one of those. Any of the ones above will work fine.
Testing fuses is both visual and functional.
Visual... Is the wire inside the glass tube intact?
Functional... When there is voltage on one side of the fuse is there voltage on the other? This is best done under load, like the fuse for the cigarette lighter, test with the cigarette lighter pressed in like your going to have it heat up.
The metal terminals in the bulkhead and bulkhead connectors are accessable in the engine compartment. If you disconnected the bulkhead connector and cleaned the metal terminals in the part with the wires attached to it and the metal terminals inside the bulkhead on the firewall you cleaned it correctly.
Some times the female part needs to have the parts punched SLIGHTLY .
Press on the red arrow part of the terminal to move the curved parts SLIGHTLY (green arrows) this makes a tighter contact to the male part of the terminal.
View attachment 1716332282
Be sure to go back and watch the videos I posted about alternators and voltage regulators.