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.
It takes 2 minutes to do the report online, you want to risk a fine and trying to argue fine points with an immigration officer who couldn't care less about your point?
then you might want to buy a cheap ticket out of Thailand within the duration of your initial entry to Thailand. If entering visa exempt then something within 30 days.