who knows, no one can predict whether an embassy will give you a visa or not based on your history. For the cash, if requested by an embassy then a statement, if requested at border/airport then cash.
not necessarily no. If you've beenl coming in and out of Thailand regularly this year then they might consider you to be living and/or working here under an incorrect visa. On the basis you requested been in Thailand for 5 months already this year I'm guessing this is your issue and a 2-3 week stint in Cambodia won't necessarily convince them you're a genuine tourist.
with a company with an entity in Thailand who will then help you through the process as they need to cut down several trees to give you all the paperwork required
a lawyer won't be able to do much for you on your circumstances. Ultimately it's your responsibility to ensure you remain legal in Thailand and whilst you might get the agent in trouble, albeit this is unlikely, it won't change your situation. Likewise with the immigration officer, he might get put into the naughty corner but you won't get any backdated stamp in your passport or any leniency.
As others have said your best bet is to pay the fine when leaving Thailand and serving the ban. If you get caught then you'll be in a lot more trouble than voluntarily leaving.