Can an American obtain a Certificate of Entry (COE) for Thailand and apply for a NON-O marriage visa afterward?

You could indeed come into the country on a 45 day visa exempt entry, get thru the 15 day quarantine, get the 400K baht into a thai bank account in your name only and then go apply for a 90 day Non-O visa based on marriage to a thai with the correct documents and bank letter

There is no requirement that the 400K baht be "seasoned" (be in the account any length of time), and as soon as you have the money in the account you could apply for the 90 day Non-O.

In Bangkok you need 15 days left on your current stamp to apply for the 90 day Non-O via (because the application goes under consideration for 14 days before you go back and get the Non-O visa and new 90 day stamped inked in)

And does a non-O visa allow working in the country, or is that an entirely separate process?

Do you have to stay in Thailand while the bank account funds are seasoning? Or could I open an account while I'm there just to have it for (possibly years) later?

You do not want to use the visa exempt option, as you will be short just a few days if you do not already have a Thai bank account in your name only. After you get out of quarantine you have just 30 days+a 30 day extension in order to meet the full 2 month requirement of your money in a Thai bank account. If you take an extra day to get your bank account, which will happen, or if one of the months has 31 days, which is also very realistic, you will not meet the two month requirement. Also, if they stop the 60 day extensions because of unable to travel because of covid-19, as March 31st is currently the last date, you will be short a few days with no easy options. Your best visa in your situation, as you are already married, is to get copies of your marriage documents, there are two, a copy of your wife's ID card, and a copy of the house book she is listed in, then you can get a 90 day type O visa based on marriage from the Thai embassy, then you have enough time to get everything together for your type O one year extension after you get out of quarantine.

Why not apply for the Non Imm O before you enter, much easier than getting it in Thailand.

No you don't need a visa. You can enter on 45-day visa exempt basis. It's tight, but will give you just enough time to get a non-O married to Thai in the country.

Do you mean enter WITHOUT a visa?