veryone is complaining about the fact that employers most of the time choose the person that bids the lowest, doesn't matter that the quality of his work is going to be
It seems then that there might be room for a "premium platform". In other words, you screen your job applicants as well as the jobs posted to make sure it's only quality work and quality pay that happens. You could even set minimums for certain kinds of jobs. If you look at the Fiverr model, they started off with only $5 gigs but quickly realized that you get what you pay for.
If people are looking for quality work, your platform could fill that need. I think that actually answers questions 1 and 2.
As for Q 3, I personally don't mind where the person is from as long as the quality of the work is good. With social media/written content, obviously the English needs to be good.