Cambodian bank accounts need to be opened on ordinary visas, valid for at least 6 months. This is easy to do...just enter on an ordinary visa (best to get an e-visa) then extend for 6 months through an agency then open your bank account in person.
Agreed...only flights are from places like Yangon, Kunming, Hanoi, Kuala Lumpur and Taipei. There are a few more but overall, not many international flights and they tend to be more expensive than flying into Bangkok.