Company Formation & Merchant Account
Also how difficult is to open a merchant account for the company (in terms of credit check) and should a merchant account be opened using our own SSN or the company's tax id?
I won't go too deep into the immigration-related part of the question, since you seem to be well informed, and because its beyond the scope of our website.
With that being said, since as an individual you can buy shares of publicly traded companies on a stock market, I see no reason why you and your friends cannot own shares or member interest in privately held companies, as long as your involvement with those companies does not violate the terms of your visa. In any case, its always a good idea to consult an immigration attorney prior to making decisions of that kind.
Now to merchant account: as fas as I know merchant service providers run checks to see the credit scores of the company representative(s), so they would need to use your SSNs for that. For more information on how to open a merchant account I would recommend you to read our article on credit card processing.