I would imagine a Bus ticket should be fine. . It's proof of your onward travel and leaving Thailand.. I've never heard of any restrictions on how you should travel.
If all your paperwork is in order then you'll get the visa anywhere you go.. the only difference between showing funds and not showing funds. Is that you'll either just have to report to your local immigration every 90 days, or you'll have to do a border bounce every 90 days. Either way you still get a year visa.
I don't know about this. But when I was there last month.. I met a guy who had traveled there from the south of the country to do his non o based on marriage. And he said that the reason why he selected savannakhet was because they didn't require proof of funds. To get the same visa that the other embassies / consulates would give him for showing funds.