No.. The DTV allows remote work only, working in any capacity for a Thai business is not exempted for work permits and the DTV does not support obtaining a work permit.
To be 100% accurate, you can 'open' a business as in the process of incorporating it, which you can do on any visa class or even a visa exempt. You cannot work in that business in any way.
No (holding any visa 'should' be enough but that doesnt mean every airline has got the memo.. I have had to have airline check in staff consult managers even with a non imm one time.
And of course immigration officers have full discretion, if they feel they dont want to let you in, they can often find a reason.
There is no waiting period. You only become tax resident after 180 days in a cal year but that does not delay changing banking etc information to start your clock.
The part of this which can cause issues is that modern KYC (know your customer for financial services) asks for utility bills or bank statements (or government correspondance), and many western banks do not like overseas residents and some even close accounts if you change to a Thai address. Secondly Thai utilities dont come in english text or with tennants names on them (internet bills do).
One step around this is to go and get a letter of residence proof from immigration as soon as you do your TM30, and then go to a certified translator to have that officially translated. If you can change bank statement do so also and finally if you rent a house and get broadband that can be easily in a western name. Those things working together usually passes KYC.