Samut Prakan is well known for being very independent and anal. Simply go the office and ask for their list of requirements. It doesn't get much easier than that
Worse case scenario is you're denied entry. Best case scenario is you're stamped in without an eyelid being batted. Immigration tend to look closer at perpetual back to back entries. Use a visa transport company
Chances are you will not be asked. I'd say 99.9% are not, but if you are, it's 20,000 baht in ANY currency. A bank statement will not suffice and there is no access to an ATM
and use either a specialised border bounce/visa run transport agency OR use a Laos border crossing (from reports it appears Nong Khai and Chong Mek are the most user-friendly). Something else to be aware of, from January 1 next year people living in Thailand for more than 180 days per calendar year will become "tax residents" and may well have to produce income/tax paid records for entry above this limit. Thai authorities are well aware that many (and I'm not suggesting you're in this situation) people are working illegally in Thailand whilst staying on tourism-based entries.
I wouldn't be basing my whole travel strategy on one ticket. Either hire an outward bound ticket through onwardticket.com or buy a cheap air or bus ticket out of the country. Some people are saying you can buy a flight to Cambodia for about 800 baht. Don't screw up travel plans for the sake of a pissant amount