Keith I think these are 2 of the easier questions and people have already answered them. The one that I was confused with was providing proof of who invited me, as I invited myself. I simply provided another picture of my passport because of this and it was approved, so don't be afraid of giving the same evidence several times. Good luck.
You could always book your return flight date for 90 days and buy a separate one way flight from Thailand to a neighboring country within the 60 day period (cheapest you can find as you will not actually use it).
I went through this process a few months ago and to save any worrying followed the advice and bought the cheapest flight online to a neighbouring country, it cost me about £20.
As James stated the 45 day + 30 day extension seems like the simplest and best solution for you. From personal experience the 30 day extension is very easy to obtain.
You can always book a cheap flight from Thailand to somewhere like Vietnam within 30 days to save you worrying. That's what I did and it cost about £20. I never used the flight.