How can a US citizen living in Mexico apply for a DTV visa for Thailand while planning to slow travel in Southeast Asia?

Anonymous participant ministry of foreign affairs hands out the visas, immigration police are at the borders and immigration offices. They are separate ministries, and typically don't know what the other is doing. That said, we don't know what's on the passport control officers computer either as far as your visa details. Nothing wrong with switching reasons for stay within the parameters in due time if necessary.

You must be in the country you're applying from, the US consulates are not naive and you'll need to provide a lot of business documents as well as tax proof. I'd apply in SE Asia. You can monitor reports from members here in the group as far as ease and requirement fluctuations at consulates. You could apply as remote worker with your online business, you'll need to draft company letterhead documents, one stating you are hired, another that you are currently employed and authorized to work remotely in Thailand and not doing business with Thai entities. A brief cover letter explaining your online business and links/screenshots to the site. You'll need to show 500K+ baht equivalent in a bank account in your name,