The more, the better, just get a rental agreement if you can.
Not sure about the embassy where you're applying. I've heard Wise statements have been rejected as financial evidence by some embassies. If the cash is sitting in a normal bank, no problem at all.
OrangeLion756 It may sound a bit odd, but one can send them a cover letter through the requested document upload. Many of us have done that to explain our personal situation. In your case it does not look very promising though, I have to admit.
Now, instead of having to apply for extensions every 90 days, you have to apply for a much more difficult extension after 180 days (or border bounce) and leave the country after 360 days. By the way you still have to do the 90-day reports. Congrats!
I don't know what a civil-law contract is. This sounds more like an employment contract to me, but I've got no idea about the specifics of your country. However, registering a business is usually not much of a pain. If I were you, I would consider registering one. Not for the DTV obviously, it's too late, but it may come in handy in the future.
Without biz registration and tax returns, it might be difficult to prove that you meet the requirements, but try to write a cover letter to explain your personal situation.