you're talking about something completely different. One, you say you had a pre-approved tourist visa from Phnom Penh so the "difficulties" you are talking about must be at the border with the immigration officer. Two, you had difficulties because of the previous education visa. Getting an agent to help obtain the visa does not help with any potential difficulties upon entry with the immigration officers.
I've used wise for years and don't need to verify transfers with an sms code. If you go into 2-step verification settings you can set it up to verify using the app (actually the default setting). It even says that's more secure.
You don't have to go before your visa expires, if you actually have a visa and not a visa exempt entry stamp. You DO need to go before your entry stamp expires though. Visa expiration and entry stamp expiration are two separate things.
Why are banks refusing to more and more? My guess is to avoid the hassle, due to increased banking regulations, of opening an account for someone to use temporarily while on vacation. Yes, you need it to apply for the in country non-o, but there's nothing preventing a tourist from lying and saying that's why they need it.
there is no law saying a tourist cannot open a bank account. Its the banks decision not to. That being said they are refusing to more and more the last few years making it very difficult without an agent.
Just Google onward ticket and use the services of the top result a few days before the flight. It costs about $15 and will give you a real booking confirmation on a real flight that will be good for something like 5 days. The service then cancels the booking after the 5 days.
then you shouldn't need anything else. People always say it's a good idea to keep your account above 400k just in case. Not sure if that's a precaution or if there are people who have actually been asked to provide an updated bank book. In the 5 years I've followed the visa help pages I do not recall anyone ever saying they needed to.