PVC is a good choice, provided that it is properly installed and maintained. A proper installation includes shielding the PVC from UV light, which is the primary means of premature aging. UV light will eventually turn PVC into a brittle mess.
ABS on the other hand tends to not become brittle with sun exposure, but to deform and is generally softer. As a result, buried ABS pipe seems to be at a higher risk of crimping, impeding water flow.
In either case, you should paint your exposed pipe, which probably wasn't done, and is the primary reason you are looking to repair it. Personally, I'd go with painted PVC, because after it is protected from UV light, it is less flexible (less pipe collapse) has a wider variety of fittings, and generally has a wider range of temperature distributions. I'm also more familiar with PVC, which might cloud my judgement. I'd love to hear from someone intimately familiar with ABS.
Both types of plastic are generally considered for low pressure applications, if you are having consistent freezing problems, neither pipe is meant to withstand routine freezing of water in-pipe.
PVC UVR is UV light resistant PVC. Buy it and paint it if you want a very long lasting PVC solution. Basically, you will have to worry about the (in my experience) natural enemies of PVC pipe before worrying about aging, that is careless yard crews, people vigorously digging, etc.
My experience with pressure gauges is that yours is broken and unrepairable. The odds of an obstruction causing your issue is a long shot at best. You may be able to replace it with off the shelf parts. If you can connect a "T" fitting to the mounting hole install a gauge on one side and maybe an automotive radiator petcock to the other side. This will give a pressure reading and a air bleed port.
Best Answer
The key to any pool regardless of sanitization or oxidation method used is to keep the water balanced. This means keeping pH, total alkalinity, hardness and total dissolved solids in-check. If the water is not balanced then you will overtime do damage to your pool and equipment regardless. Well balanced water should not be corrosive. There are lots of other metal parts in your pool other than your lights - ladders, heat exchanger (in a gas heater), etc.
Your lights should already be sealed, if they were not, you would be losing water through your lights.