I personally created a legally created ADU or mother in law unit. 

And that's why the "due on sale" clause is not a very big issue.  

And, if I buy it with the LLC immediately, do my in-laws need to have a rental contract or can they just live in it?

You'd have to find a person familiar with MN tax laws to address how the CA-sourced income would be taxed on the MN returns.As to the LLP/LLC question, LLPs require a general partner, which usually has unlimited liability. 

There is a state law from 2014 where municipalities cannot create new or amend previous laws regarding how long or short a homeowner can rent their property for. 

I am not sure why is that but you have to be careful on what are the agreement on that contract.In this case, I would recommend to get involved with a investor friendly realtor who will help you as it could be many issues during the process as the titles are not clean in most cases and many variables have to be checked. 

I'm sure several here has or will have the same issue in areas like mine, there are many multiunits out there. 

The real issue is what is done with any overages.   

Is there a Realtor "law" that requires a listing agent to submit any offer to the seller?

As a Real Estate Professional, I know I have some friendly Tax Laws that can help me but I am unsure where to look.