The embassy might or might not change it. The simplest thing is to just carry both passports with you and hand them both over to immigration along with a printout of your DTV in future
Nobody can say definitively, only guess. The same as you. Does your partner work remotely from Thailand? If so they need to get either a DTV based on workation or an LTR Remote Worker visa as these are the only 2 visas that allow remote work from inside Thailand.
you said you entered October 2025 and put money in your bank November 2025. An educated guess says the agent got you a 90 day Non-O and 12 month extension at the same time. So therefore were you just to go for an extension you would be able to show you had the required funds in your account for the duration of your extension. Whether another immigration office would deal with you considering it is not a strictly legitimate stamp
the rules do not say that. The rules do not specify any limits by either air or land. But they have been cracking down on visa exempt entries in any case and more than 2 visa exempt entries (including the one you originally entered on) can be a real problem
that's the cleanest way to do it but because you can show you have had over 800k baht in a bank account for over 12 months you might get away with just doing an extension if the don't object to the iffy stamp