You have mentioned on #3 that both of you have International driving licence! You have been driving without licence and I am pretty sure no insurance will cover that.Anyway, I liked Koh Tao so much that I am thinking of spending 2/3 months there during wintertime in Europe. I will get an international driving license and make sure that I have full coverage with my insurance.
Never mind because both of you won't the only people to make such a naive decision.
If you like diving then the wintertime in Europe should spend on the West coast of the Kingdom eg. Koh Lanta, Koh Lipe etc.