technically you have income of 800k, which cant be treated as a gift, and unless it has supporting documentation for a structured loan, term, interest rate etc cant be treated as a loan. You have no tax credits from your home country for it so the 800k is assesable income and subject to tax. The first time maybe ok based on your tax residency, but the second year and subsequent years you are probably a tax resident.
that should be a last resort, as your paying them big bucks to borrow the money for a day, not adhering to the terms of the extension, and have to repeat the visa and extension process the next year via an agent, and you could owe the government upto 240k in tax everytime you do it.
Personally i would ignore the fact you have a work permit and carry on using the 400k method (unless you have problems holding that), its a lot less paperwork.
your confusion is that the stamps you get for the extension are done by one team, and it has the standard wording about requiring re-entry permits, after this was given, you elected to have a second team issue you with the re-entry permit. So you are covered, nothing more to do.
If you've got 400k+ baht in the Thai bank ( for 2 months prior to application), just go that route for renewal, just using letter and statements obtained from bank ( no older than 7 days), plus copies of bank book upto and including application date. Dont go down the work permit route if you can help it. I'm married and have a work permit, and my employer organizes my visa renewal every year, and we always take the banked money route as much less paperwork.
Legally no, if your returning to same address within the check-in period. So your breaking no law by not doing it again. But if you have to interact with an imigration office in person many like to see an updated one before they will sign off what ever you went there for.