depending on what your business is it might be protected for thai people only. You should consult a lawyer, you'll also need a minimum amount of money invested to establish a foreign owned business 2-3million baht depending on catagory and thai employees if you want a work permit from your own company.
15k usd isn't 500k thb you'll need more if it's in foreign currencies. Should put more then enough in the account if it's not in thb to account for currency fluctuations.
No because 15k usd isn't 500k thb. That's currently worth 481,800thb. You should have more then enough in the account to cover fluctuations in conversion rates.
That's awesome welcome to thailand! You're good for 180 days, but make sure to do a 90 day report in the middle. Alot forget or don't know to do this on their first trip.
You shouldn't lie to your company about the remote work destination. Depending what type of company you represent they may have contracts with data privacy restrictions for access from certain countries etc and you would put them in violation.
Nothing changes for DTV or any visa holders. All the immigration changes are in regards to visa exemptions. The changes have been made to crack down on illegal working and scam center operators mostly.