Simply write a brief cover letter explaining that you are transitioning from one job to another, hopefully it's within the same field of work, you can supply your previous pay slips/bank statement to show that you have qualifications and experience in that area.

Not an issue, just put the initial accommodation details.

Minimum 6 months, some embassies are fussy about the length of time.

As mentioned do the 400K transfer after the 3 month after stamp seasoning and before the 2 month seasoning prior to extension application

You applied a few business days ago and were expecting an immediate approval?

A brief cover letter explaining your job transition and lack of current job income, previous job deposits will show you have experience in the market.

They don't sell the 1 year non O any more. You buy the 90 day visa and then fulfill the requirements for 1 year extension before the 90 day stamp expires.

Be aware that you're Vietnam entry Visa is likely in your old passport and Vietnam's pretty stickly for seeing all the pages of your passport so I suggest you show all the pages of your old passport and at least the first few pages of your new passport which your application will be done under. Suggest a brief cover letter explaining the passport situation.

Document requests reset the processing clock.

Actually printout the entire visa PDF sent to you, not just the QR code.