But remember that if you pool it all in ONE account, you will then need to wait 3 full months to submit the statements as initially the account you chose will not have had the required amount. People get rejected on their marriage or retirement extensions for exactly the same reason, if they dipped below the needed 400k/800k even one day in the middle of the 2 or 3 months period before submitting.
Dive shop can provide what they want, it still is not officially included in the soft power wording of the law on official ministry foreign affairs sites. Dive shops can have their own incentives to get you to do the DM course, so be careful not to take what they tell you as gospel. Best check with an embassy or consulate where you will apply if this is really the way you want to go. But I would be extremely surprised they accept it -- as it has not specific relationship to Thai culture itself, contrarily to muay thai and thai cooking or thai arts...
Call HCMC embassy and ask them if you can. By default, I would upload the 3 latest official ones + add a screenshot of the upcoming statement (to make 4 months).
Of course this is only an opinion but I think if you can make up some nice plan for the next 5 years during which you will supposedly be in/out of Thailand taking multiple regional thai cuisine courses and also going to other neighbouring countries, doing similar things and also going back to Belgium. Bla bla bla. You don't have proof of further courses, as you want to discover and will make more solid plans as you go along and as you progress in your art.
Apart from giving an opinion ( at worst case they will ask you why the discrepancy+ ask you to correct it), why don't you just email or call the embassy ? Tell them you messed up and send them a new cover letter with the corrections. They can't fault you for trying to correct a mistake... When I submitted my documents, I had a few friendly with a lot of "kaaaaa" back and forth with the staff to provide additional info and all went well...
I would submit all the different account's statements over last 3 months as you are required to show the money has been there for 3months or more. If you transfer all into 1 account, you will need to wait 3 months to submit your statements. In case of doubt, call the embassy to ask + explain.
I am not sure what you want to know, as it seems pretty clear. If the bold text is from the embassy, then ... you need to provide the business registration of the company you work for (usually legally operated companies can be found on an official government website from the company's country... so if it's a UK company there is likely proof somewhere on a UK govt site that your company is registered, a registration number: find that and give it to them and also provide a link to that specific website... NOT the company website!!)
And as others have stated, your company needs to provide a letter specifically stating you are allowed to work remotely FROM Thailand (and you can add any other countries if you want to use this letter for other countries...).
I don't think the requirements have "changed" so much, but some people have had very different experiences with the IO, especially for the ones on the "soft power" application; experience has shown that many thai schools have very lax criteria for attendance checks... There has been a lot of discussion (info can be found searching in Thai media) about installing checks to ensure attendance and progress for all these soft power courses... which makes it IMHO more attractive to apply as a remote worker/ company owner if you can qualify.
Finally one very important thing to remember: the visa is not an absolute right to come into the country. The IO in front of you is the sole person who will authorize you to come in... or block you.