you can't do that to apply for the initial 90 day Non-O visa as when you put the money in a bank account it won't show as coming from overseas. For an extension of stay you could do it.
Simply saying you have a bad back is meaningless. You need a treatment plan signed off by a recognised doctor at an official thai hospital with official stamp that's at least 6 months in duration
there's no requirement to show the money is from overseas for an extension based on 800k baht. You still need a letter from the bank but it's a different format. I always just ask for a letter for immigration and my bank understands what's required