I'm pretty sure if you visit 20 to 30 bank branches, you'll get a bank account sooner or later. With good planning and a motorbike, you can easily visit 20 branches a day
I understand your frustration, but ultimately you have no choice but to wait. You have decided to spend a "holiday" waiting time in Bali and not in Jakarta. If you were in Jakarta now, you could simply go to the consulate and ask about the status of your visa application. The uncertainty is the price you have to pay now
If you can prove that these were your investments and that you converted them into cash, then it should be fine. The three or six months of proof of ownership is basically just about making sure you're not borrowing the money from friends or an agency. But you could clarify that with them in advance. I think you can talk to them, since you have the money and it belongs to you.
If your Muay Thai gym is in Bangkok, then I would definitely not include any travel details about other regions in Thailand. I wouldn't even know where you'd want to include or upload them. I would keep it as simple and straightforward as possible. The consular officer doesn't want to read through your travel plans or letters because they're stressed out and have to process 20 visa applications a day. Furthermore, if you overdo it with documents that aren't required, they might suspect that you're trying to cover something up.
It's not required to include such a cover letter, but if you absolutely want to, include it under point 5 along with your Muay Thai documents. The cover letter should then only be about Muay Thai, i.e., your intention to learn, train, and improve Muay Thai.