as with everything else here it depends. On what office. On what specific officer. On what day of the week. On what color socks your had on last Tuesday. Your experience in Sisaket may (and will) be different from the experience of others. Both in Sisaket, but more importantly in other immigration offices. Best advice is to check with your local office as advice from a different office might be useless.
have you ever visited a Thai government office besides immigration? That's just how it is. For the Thais too. People in different offices are sometimes queueing outside for hours and hours. Bring some shade, and make use of the always present food/drink vendor when more than 5 people are gathered. 🙏
immigration most likely will ask questions. But you can cancel it before time. And you are allowed to be a student on any kind of stamp/visa/extension. So just make sure to cancel it before you leave. Ask the school for an updated paper with your date of departure. Then 1-2 weeks before that go and cancel. They will cancel your stamp and make a new one valid until the end date mentioned in the paper from the school. That way you don't have to hurry with leaving. Amd if you dont cancel it you can gwt fined 20K baht in the future if trying to get another visa/extension in Thailand, for failing to cancel it.
students at my uni generally has to border bounce to tide them over until they can get the degree after graduation. The uni can help by not putting in the registration of grades etc until last minute. But that practice is questionable at best..
You most likely will have to do a border bounce very soon. To get into Thailand on another stamp or visa that lets you stay until you can figure out another long term visa if needed. It's not possible to change the ED extension to anything else in country.