So recently started getting a P0128 code, which my understanding is due to ‘likely’ the main thermostat. I saw this video on how to replace it: https://www.youtube.com/watch?v=10aySOX7aV8

Now, here is what I know and what I don’t know.

KNOW:

• My valve cover gasket needs replaced, as I’ve encountered oil leaking from it and I know into spark plug location #1 (far left).
• Oil levels recently were quite low, and I suspect the engine is eating much of it. In the meantime I did a full oil change just to at least get fresh stuff in there
• My coolant reservoir looks fine. I topped it off the other day, but it was above the ‘min’.
• The engine temp from inside the car always appears fine

NOT KNOW:

• Can the coolant be low but reservoir be ok?
• I need to do some driving this week that will be in the 2-3 hour mark. Would I be ok or should I consider other transportation until I get the thermostat replaced?
• Could I take the radiator cap off and add more coolant there? about 4-6 months ago I replaced the oil cooler gasket, which required me to add coolant that had been drained, but I didn’t drain the coolant via the drain screw under the car, just what all came out via the oil cooler hoses (2 hoses)

Basically, am I at the stage where any driving can harm the vehicle?