The first thing to note is you can't get a DTV visa inside Thailand. You need to apply at a Thai consulate and there aren't any in Thailand so be very wary
If you are having to make a new visa that will be entirely down to the new company. Most companies want you to get a new Non-B visa outside of Thailand as the paperwork they need to supply is much less challenging
I guess you mean a visa exempt entry. The extension process is straightforward so no need for an agent. And while you're at immigration ask for the handout of requirements to convert
If you were to get one the other would be rejected so chose the one you want and go with that. The Non-O seems a much better visa to me for long term stay