Cambodia can get very hot and uncomfortable in March, though early in the month could still be bearable.
I'm in two minds about the flights I've always taken non-stop flights to Bangkok but they usually seem endless. I did take a flight from Hong Kong to London once that stopped for a couple of hours in Bahrain and though the overall time of the journey was longer the actual flights seemed more bearable as I am forever looking at my watch and saying to myself another ten hours.. nine hours..etc until I land, whereas saying another six hours then five hours until landing doesn't seem too bad..
I think it can also depend a lot on the airline,if you are flying economy Thai have good legroom so the flights are more comfortable, BA economy is cramped as is Emirates but with the stop over their flights don't seem too bad. I haven't flown with Eva who I believe have a non-stop flight to Bangkok but I think they have good legroom. I think if I was visiting Cambodia again I would try flying via Kuala Lumpur with Malaysia who I rate very highly, missing out Bangkok entirely though only because I had an enforced stay there at the end of November.

Have done a change at Dubai before and agree with Judith. Although it added 2 hours to the journey it actually felt a lot better as the longest we had was 6 hours in the air. I can climb the walls in a long flight. However, I do just go with the cheapest option.