The financials for applying for a visa from overseas can be in any bank in their name only. Currency exchange has to be equivalent or greater than the Thai baht value listed.
That will depend if you are actually on a visa (the initial visa either gotten in Thailand or a visa from an embassy abroad) or you have a stamp that says “extension of stay”.
If the former then you can stay until the date you were stamped until.
If the latter then the moment the reason for the extension stops then so does it. You must leave the same day or switch to another kind of visa/extension if you qualify.
No issue coming back. What may happen is the next time they do a 90 day report an eagle eyed officer may spot they didn’t do a previous one and fine them 2,000 baht.
The gym won’t require any form of meet up prior. You contact them to book a course. Pay the fee and they send you documentation to use with your application for the DTV.
Whether you actually use the gym and attend classes is almost irrelevant.
If you came in in March then you’d have been given a 60 day stamp.
They haven’t changed it back to a 30 day stamp although there is speculation that they may do so but regardless of whether they do or not is irrelevant. You have a 60 day stamp that is valid for that time. You can also extend that one time for a further 30 days at a local immigration office.