Unless you already have a Thai bank account, getting a retirement option inside of Thailand is not an option.
Do things the way they are meant to be done. Get the visa you need from a Thai embassy before you travel, and that will allow you do the next steps inside of Thailand that will allow you to extend that stay.
The same thing would happen ANYWHERE if you don't have the correct paperwork to do something official with the government. Government agencies in America will happily send you away after waiting months for an appointment if you don't come prepared.
That's completely dependent on your immigration office.
Most require you to file a new TM30 when you leave Thailand and return, even if it is to the same address.
Some don't care where you go inside of Thailand as long as you return to the same address, but some require you to file a new TM30 even after domestic travel.
You can just have your landlord create an online account and send you the login information and you can do it yourself.
Where you enter is irrelevant. If you qualify for visa exempt and enter without a visa, you'll get a visa exempt stamp if you're admitted into Thailand. I don't know if you'll get one since you don't say what passport you have.
That's not normally a requirement for embassy certified income. That being said, each immigration office is different. So only your local immigration officer can tell you whether they'll accept your certificate or not.
The TM47 90-day report is your responsibility. You must keep track of when it is due. If you leave Thailand and return, your next report must be done in person. This is the rule every time you leave.
1) the 90 day report is supposed to be so they know where you are every 90 days. So you're supposed to do it where you are physically at. You would need a TM30 to have been done in order to do a TM47 there.
2) TM30 has nothing to do with a lease. It's the legal requirement of EVERY property owner in Thailand to submit a TM30 within 24 hours of arrival of any foreigner staying at an address.