Yes. However it’s usually always better to have the 800k as a backup when trying to switch. Sometimes things go tits up with a single transfer and if you don’t have the 800k then you have no option other than to leave and start the process again.
If you have the 800k for 2 months then you can use that and get all the ducks in a row for the next years application.
Get a letter from the hospital doctor saying you are unable to travel. You take that (or someone else if you can’t) to immigration and apply for a medical extension.
Depending on what the doctors say it can be granted for up to 90 days.
Very difficult to do. Copious amounts of paperwork to provide. It can be done but most just opt to bounce.
For an example I know 6 people in Phuket have managed to get an extension so far. One person took around a week and numerous visits to immigration but finally got it. He commented to me that next time they’ll bounce.
If your country passport qualifies for visa exempt then you’ll get 60 days on arrival. You can extend that once for 30 days.
As to how far in advance you can apply will vary depending on the office you use. Some like Bangkok, Chiang Mai, Phuket and Buri Ram (perhaps others) allow 45 days prior. Most allow 30 days. Some only allow within your last 7 days - Jomtien, Koh Samui and Koh Phangan
I am not aware of all types of bank accounts that the many branches could offer. Some may well offer fixed term deposits for x number of months with no access to funds until you meet x.