you don't understand the difference between tax residency and tax liability. You never lose your liability for taxes in USA even if you are a tax resident in one or more other countries. You are wrong.
In theory, if you moved here and deposited before July 1, you would be a tax resident in the current year and could be liable for tax. But it depends on the source of the 800k. If you can show it came from social security then it wouldn't be taxed according to the DTA with USA. Easiest is not too move till after July 1, then you won't be a tax resident for 2025. Given your writing this on jun-30, assume you won't be a tax resident.
If you're coming in a non-o, you'll basically need to leave the 800k in a Thai bank for 14 months (technically for 3 months following your first extension with balance not taking below 400k, then back to 800k 2 months before your 2nd extension. If you have 12 consecutive deposited of 65k prior to your 2nd extension you can convert to the 65k/month method and can take it the 800k.
No requirement to be in home country when you apply. But do need to show that you're not in Thailand (entry stamp to other country, hotel invoice, etc).
Not going to happen unless she's from a wealthy Thai family and has business ties to Thailand and significant assets in her name. Unless you can buy her a Trump Gold Card. You might get a fiancee visa but it will take months and you so need to marry her within 6 months of her arrival.