You are, as Americans say, barking up the wrong tree. You cannot sue for visa denial. And there is NO just cause for giving you a US visa just to file an income tax return. You can communicate with your corporate attorney via phone, email, fax, etc. Your corporate accounting firm has to do the actual return, and can likewise communicate with corporate attorney. Everything is done online, and you have NO reason to travel to US. Get going with corporate attorney & accounting firm and have them get it done.
Need physical signature on the return? Ever hear of FedEx, UPS and DHL?
You have NO base to sue. Your US partner can hire any US based accountant to file taxes for him. Why not US partner have same LLC tax accountant do the same work for him in the US? Why is it so important that you have to be here? I don't get it. Does not make any sense. It is totally useless for you to be there. Thats my point. Again, there are zillions tax experts who can do it for him in the US if needed.
My advise, please do not waste your time, money, energy on this.