Will my Non-O visa remain valid if I leave Thailand for a week and return?

You will need a Re-Entry permit (two options A Single Re-Entry permit or A Multiple Re-Entry permit.) I'd recommend you get it from your local immigration office, though you can get from immigration at the airport.

Yes are long as you have rentey permit

Yes it starts from the 15 September you be ok speedyAsparagus5533

If you have multi entry (3,800 THB) no problems.

If you buy a single re entry and your back in time with 2 months of seasoned funds you’ll

be fine

Brandon has comprehensively answered this question. I would get your bank account opened and your 800,000 baht transferred in that first week, to avoid a big rush later, especially if there are any delays in returning.

When you enter Thailand and use your visa, it becomes invalid as of the date you enter, because you already used it.

That non-O visa will grant you a 90-day permission to stay in Thailand. You're actually asking "will my permission to stay still be valid?"

You must purchase a re-entry permit before you leave. This keeps your permission to stay stamp alive if you leave. Were you to leave without a re-entry permit, your permission to stay is also invalid as soon as you leave.

With the re-entry permit, as long as you're back in Thailand before your entry stamp expires, you'll be stamped back in on the same stamp with the same date, and you'll then be able to apply for the annual extension.