Not every location does this though and then to to every establishment. Genuine students have never had issues. Those who just want the visa and not attend generally choose establishments where this is not an issue :-)
Certain locations this has been true a while though. When I did the driving test the other week one guy was saying his attendance was checked and had to be taken to immigrstion for extensions. His school would not lie for anyone. They also had immigration visits unannounced.
No idea about his application but just yesterday a guy from UK posted about being in Thailand while his application being processed in UK. The Embassy certainly can see a lot of information is the system. Whether this is total record who knows. If an Embassy puts a note in system why they rejected DTV other embassies can see it. As for your poi t about denial at point of entry not seen a credible report yet of anyone denied entry while holding the DTV.
The last 3 places I have rented since 2019 I paid the 2 months balance at time contract was signed and handed over. I had paid 1 month rent to reserve the property as all three were in demand. The one I am in now I did not actually view as so many after it. If you pay the deposit to reserve then yes the agent should give you an immediate receipt.