There is no official requirement to keep the 500k in the bank. The only time it is required currently is for in country extensions and for 30 days before application.
Look through previous posts.. This is asked every day. Basically no. Personal bank account. Their might be a rare embassy or Consulate has accepted investment etc
Search through the posts. Dozens and dozens have done it. About length of course some Embassies want to see a longer course ie 6 months. If it is shorter some Embassies will issue a METV instead of a DTV in some cases.
The DTV seems not to be a good fit for a start up idea. Maybe certain embassies woukd support it. As the other poster days look at BOI Smart S. If not why not just do Soft Power and still work on your startup. Good luck either way.
Quite a few expat groups reporting this. In those it has been to get a certificate from your Telecom confirming your number is tied to your ID ie passport. There can only be one number per account and has to be in your name. If you have a new passport that number needs updating at both bank and telecom. Hope it is just that and not due to visa type change.