Confused and need some ideas on what to check. For context I drive an 07 TD42T swapped GU.

Situation is I was driving from Sydney down south on the Hume Hwy and noticed that about 2 hours in the coolant temp kept dropping on my redarc gauge to around 68-70*C at 110kmh and would peak about 83*C on hills - normally runs around 80*C peaking at 90-95*C on hills - was normal for the first 2 hours.

It’s been a cold and rainy few days and in the morning the temperature refuses to rise even after 10-15mins idle it was sitting at max 30*C until I start driving.

Today (18*C) on my lunch break I let the car idle after driving back to work and the temp kept rising and rising until I switched it off around 95*C to go back to work.

I assumed I had a faulty thermostat so have ordered a genuine 82*C thermostat and plan to replace and do a coolant flush this weekend. But now I’m confused as if the thermostat was stuck open why would it rise so much at idle today?

So 2questions:
Any harm in driving it back to Sydney on Friday - what temps are too cold and too hot for a td42t running 14psi
And
Any other ideas about what’s going on