"MERS could not have possibly assigned the mortgage from Lender 2 to Lender 3.  After MERS assigned it to Lender 2, it no longer had any interest in the mortgage to assign."

Therein lies the crux of my arguement Robert, since you do not know the servicing contract set up with the original lender nor the "chain of title" within MERS you cannot assume MERS did not 1. re-purchase the note, 2. have the note reassigned back to them for non-performance or if it as you say 3. outright error. 

MERS has over 170 some thousand notes in their portfolio and handle an average of 5k per month in and out, there is a chance a problem is there, but what is the likeliness this problem has not been addressed before and they simply chosen to not correct it. I imagine there is a simple solution other than MERS just decided to sell or transfer a note twice committing a felony act.

MERS is a database and clearing house for notes, review the chain of ownership through MERS and determine how this cloud came to be. If the problem lies within the MERS chain, best to go to the source of the problem.