I'm not sure it's even possible to cancel a visa. But you can convert a DTV to another type of visa at an immigration office inside Thailand. You will need paperwork from your employer to make the change
if you are over 50 it makes sense to get either a Non-O or a Non-OA visa particularly as you own a condo. Personally I'd get the Non-O but the choice is yours
You need to have a job for the LTR option you propose plus meet the other requirements. You can't work in Thailand on a DTV. Seems to me your best option is to find another job that will support a Non-B and Work Permit
correct. He know lives in Thailand, he is just another tourist and can only stay for 180 days at a time, he may well lose his bank account due to being a tourist, he can no longer get a 5 year driving licence etc.
As well as having the 500k baht in the bank it is increasingly asked how you intend to support yourself for the 5 year duration of the visa if applying using soft power. Think carefully how you respond if asked