If you don’t have any recent excessive travel history on visa exemptions, you should be fine with a re-entry and still be considered as a genuine tourist to Thailand.
You should be able to a apply for a Non O in country on a TM.87. It’s known that some smaller immigrations don’t offer this service. Where du you stay?
No, you just have to show twelve consecutive transfers of min 65K baht a month from oversea to your Thai bank account. The problem with changing accounts for the 800K baht is that you’ll also need to prove that the 800K baht stayed in your account for three months after the extension was issued and didn’t fell below 400K baht within the twelve next months. Therefore it has to be the same account as you where approved with.
I’ve seen feedbacks of people getting denied this two months visiting Thai family extension at other immigrations too, but there are for sure no formal change in regulations or a new police order regarding this.
For a reason I don’t know they’re getting stricter on this now and I’ve seen same feedbacks for Khon Kaen and Udon Thani. And they didn’t ask for a bribe, they’ve just stopped issuing them.