No you can't use the pressure switch, as it is just that, i.e. a switch!

For a gauge you will need the correct sender which is calibrated for the gauge, this then needs an adapter fitted ideally so you can have both the light and the gauge working.

Re the lights, if they are on permanently then they have a power feed going to them, I would pull the light fuse, this should extinguish the lights, if it doesen't then you have a feed from elsewhere, it's almost certainly been caused by someones ill informed meddling.

Earthing issues can cause problems but not like what your describing.

If you can isolate the problem to the light circuit then it's far easier to resolve the issue!

Martin