I'm looking into getting a certificate as well as doing a program at an international Thai university. The program would last 10 weeks (120 hours) and the certificate would last 2 months (144 hours). 

 

- How do ED visas work when not doing long-term studies (or when studying multiple programs)? 

- How can you lose your ED visa?

- The certificate very probably won't start right after I finish the program, or vice versa—can there be days or a few weeks of not studying without losing your ED visa?

You could do it on a tourist visa too (60 days) and extend in the country for 30 days. Then leave to a neighboring country and get another tourist visa, extend if needed.

You would first get a single entry non-ed visa that with paperwork from a school/university that would allow a 90 day stay.

Then a 90 day or one year extension of stay at immigration if you have the supporting documents to get one.

might be worth talking to the Uni and the school. I believe they are usually pretty clued up on how their foreign students visas work and the requirements etc