you can book 5 nights. A cheap hostel is like 120 baht per night.
Landlords have to pay tax on rental income, and Airbnb is illegal, so they can't pay their taxes, which mean they'll get into trouble if they report you staying there. At least that's what they're afraid of.
if thai immigration rules are giving you a migraine (which is very common), take a look at the Philippines. No visa issues there, just enter on a visa exempt stamp and stay up to 3 years at each entry 👌