I believe it needs to come from the district where you reside. But your residence can be the last place you stayed. When I did it they wanted copy of the lease agreement along with signed copies of the landlords Id. I got certificate immediately by paying a couple hundred baht for expedited service. Don't think that's available at all offices. You need a copy of the registration of foreigner residing at the same location as the lease agreement.
I don't think you can get a reentry on the initial 90 day non-o, only on the 12 month extension. If you leave, you have to start over until you get your 1st extension.
Assuming your embassy doesn't provide income verification....
฿800k in Thai bank for 14 months (yes, you can draw it down to ฿400k and put it back). During the last 13 of those you can deposit ฿65k/month from foreign source. Applying for your 2nd 12 month extension you can use the ฿65k/month. You can then use the ฿800k however you like.
I've heard this could be immigration office dependent, but it is how the law is written.
I found that when using swift, it is best to send money to Thai bank in your country's native currency. Difference between what my us bank gave and what Bkk gives is significant ฿2-3 per $. My bank also charges $65 per transfer. So for under about ฿75k, wise ends up cheaper.
Go to onwardticket.com. get a reservation from Bangkok to phenom phen for about $16. Do it the day before you travel. The reservation is valid for 48hrs. U can just let it expire. Should be for within your first 60 days.