Eviction question on illegal unit

[Wilson Lee]

Hi BP forum, we have a multi- unit in California (LA county) and just found out one of the units was illegal (city report performed inspection while trying to sell the property). Now we're trying to get the tenant in that unit to move out without starting any formal eviction paperwork. They have lived there for over 20 years and unwilling to move. We've tried cash for keys and also offered to move them into a larger vacant unit on the property for the same rate they are paying now. Win win, right? But they are being stubborn.

Our issue is, when we start the eviction process and eventually evicted, can they sue us for the 20 years of rent collected? We're received 3 different answers from 3 different attorneys.

Attorney 1) Yes, they can sue you for 20 years of rent paid. But it is up to the judge to deem it reasonable.
Attorney 2) No, they can only sue you for up to 4 years due to statute of limitation.
Attorney 3) They can only sue you for rent paid from the time the city reported it as illegal unit.

So which is the right? 

Also if anybody has a good real estate/eviction attorney in the Los Angeles county area they can refer, please drop me a message.