1- No you did not
2- Yes it suffices
3- You were not wrong
4- Your obligation is to know whether the owner is Muslim is not. You don’t have to investigate anything else.
5- It’s not about the number but rather who is in charge of the market. If Muslims generally are in charge of the market and generally decide where the meat comes from then you can take from those butchers. Otherwise, if this is not the case, you can only eat meat from the Muslim butchers.
6- You do not have to ask.