many over 70 (typical expat on a non-o) will find health insurance very expensive with many exclusions. If over 70, expect to pay about ฿15k/month and may find that many issues will be excluded as preexisting conditions. Insurance here is archaic.
The point of the 800k is that it is to be accessible and liquid for medical (and other) emergencies. I don't believe a fixed account would qualify since the funds aren't available to you.
I'd talk to a lawyer since a non-o does not give you the right to work .. as you've already been advised, it would be best to twin and open a bank account in your home country and remit funds to Thailand as needed.
Health insurance here is horrible. The older you get the harder to obtain and cost can be prohibitive. Bring health records with you (and all legal documents like divorce decrees, etc). Health insurance doesn't cover or existing conditions. If you take medication to control blood pressure, make sure whatever policy you by will commit in writing what will be excluded as a result. High BP and diabetes (or ore-diabetes) will exclude many other health issues related to organs. Good luck.
I'd move the money and show statements from both accounts showing the movement into your name and the length of time the money has been in your brothers account.
Just curious, if it is indeed your money, why is it in your brother's account. I think embassy will be suspicious of the real owner of the funds.