If the lawsuits are discharged due to the borrower not signing,them Prosper has to pay the lenders back..
That's pretty simple! 
Actually, this is 100% correct. Paragraph 7 of the LRA provides:
7. Representations and Warranties as to Notes Sold. Prosper makes the following representations and warranties to you that, with respect to each Note sold to you under this Agreement, as of the date the Note is sold, assigned and transferred to you:
* * *
c. Prosper has made commercially reasonable efforts to authenticate and verify the identity of the borrower on the loan evidenced by the Note. Based on such authentication and verification, to the best of Prosper's knowledge: (i) the borrower had full legal capacity to execute and deliver the Note, and (ii) each Note sold to you by Prosper is the legal, valid and binding obligation of the borrower, and is enforceable in accordance with its terms.
Thus, if Prosper's Prommissory Notes and/or procedures are defective, such that a Note is not the "legal, valid and binding obligation of the borrower, and is enforceable in accordance with its terms," Prosper has breached this warranty. And Paragraph 8 of the LRA specifies the remedy for such a breach:
In the event of a breach by Prosper of any of the foregoing representations and warranties that materially and adversely affects your interest in a Note sold to you under this Agreement, Prosper shall either (i) cure the defect in the Note, if the defect is susceptible to cure, (ii) repurchase the Note from you, . . . . In the event Prosper repurchases a Note, Prosper will pay you a repurchase price equal to the outstanding principal balance of the Note as of the date of repurchase, plus any accrued but unpaid interest on the principal balance as of the date of repurchase at interest rate set forth in the Promissory Note."
If Prosper's Prommissory Notes prove defective, that would be the best thing that ever happened to lenders -- Prosper would have to repurchase all the defaulted/late loans, at full value including accrued interest!