1. You can only hold 49% ownership of the business.
2. You have to employ 4 Thais for every 1 farang.
3. Your partner must have a financial investment in the company, equivalent to the share he/she owns.
There are some exceptions such as industries promoted by BOI. Also, if your partner is your spouse the employment rules change (I believe but please check).
It really depends on the airline. If you don’t have one then they have the right to refuse boarding. I have been asked on my last three trips, different airlines each time.
Yes, I imagine that they do. It would be nice to know which! Never seen it posted here but I’m sure there’s someone with experience of it that can help.