In the past, a lot. These days, I'm not sure. It's not a good option for long-term stay. Fine if you're a genuine student to get through the first 6-12 months in the country.
True, but I think as long as you're actually attending such courses, all is well. I can totally see Thailand NOT granting visas for such courses, for longer than around a year.
Regardless, I can't see a Thai bank issuing a credit card to a farang without a work permit.
Also, if you're talking about a secured credit card, what's the point of that? Just use a debit card with the VISA or MasterCard logo, works almost identically to a credit card.
Only reason to use a credit card is if you actually avail yourself of the credit facility, otherwise, it's not really a credit card anymore.
Not surprised that everything went well, that is to be expected on a DTV. I'd advise printing out the DTV approval letter even if showing it on your phone was accepted.
Leave it alone. Nothing you need to go into the bank for unless you're renewing a passbook (which unless you need it for immigration, which on a DTV you won't) you don't need to bother with so that leaves only applying for a new debit card, which is once every 5 years.