Some offices require you to hold a Thai bank account and deposit the 500K in there prior to seeking an extension of stay. If you can't open a Thai bank account or no longer hold one, you won't be getting an in-country extension, thus the borders being necessary.
Just bring cash, a debit and credit card (preferably 2 of each). The ATM user fee sucks, but unless you hold a Thai bank account or a card that refunds this fee (or a trusted local with a bank account you can transfer funds to, such as your wife or children) you'll have no choice but to pay it.
Is there something wrong with you? What's wrong with stating obvious facts? Backpackers catch Air Asia flights! Nothing wrong with that. Only you seem to be in denial.
How do you identify a "backpacker"? I'm sure there are western backpackers on every Air Asia flight save for one's going to places like Khon Kaen or Ubon Ratchathani.
Yes indeed; Cambodian borders are closed to all travelers. No trade going in or out.
Ranong-Kawthaung is open but the Thai side is strict - they want to see 20K in cash, a hotel booking and ticket out of the country. You might be able to talk your way out of some of these requirements if you're fluent in Thai and you're not on a visa exempt stamp.