Ian Hay. You have much to learn about Thai culture. It's not about wanting people to spend cash. If this was the case, they would not now be kicking out the "covid extensioners". Thai immigration do not give a stuff about people spending money in the country
Apply for an extension on April 11, and you'll get a "seven days to leave" stamp to the 18th. That's about it. Otherwise you'll have to do a visa run to Phnom Penh. It's possible that some land border MIGHT be open by that time, but you probably MIGHT have to do the Thailand Pass re-entry. It's still
Your employer will assist. There's documents your employer will provide which will enable you to get a work permit, from which you can then convert to non-B visa
No chance of getting a card while you're outside Thailand. It's a monumental process getting one whilst you're in the country:- personal appearance at your bank branch with passport in hand!
You can withdraw 400k baht three months after your extension is granted. Keep the other 400k in the bank until the EXACT day the previous extension expires, or you will not have met the condition of that extension.