I need advice for creating the lease to include guidelines and wording on sharing responsibilities in the common areas. 

Any law that tries to do otherwise conflicts with sb13 and ab2533 (unless the unpermitted units was created post dates of protection by those laws).i agree he should get it permitted especially if close to the price quoted, but issues due to unpermitted units that are protected be sb13 and/or ab2533 present minimal risk and if it does happen get HCD to assist in any issues. 

Now the value set be appraisers typically is coming in between $50k and $100k   I invite you to search BP or create a post determining values that appraisers are giving to ADUs.  

2.Create a direct booking website and set up an escrow account where guest payments are held and disbursed to the property owner and cohost according to agreed terms?

Can I just fill out a grant deed myself and create a loan document and have them both recorded together?

Check out FilePlace, where user-created forms are available for free.

Your dilemma boils down to two priorities: maximizing your investment growth versus creating financial stability for your personal residence.

Quote from @Jack Krusinski: When my rental properties are in a situation like this, I make an effort to create a structured and aggressive payment plan to help tenants catch up.

If you haven’t already, consider learning underwriting software or creating a detailed spreadsheet model to speed up your analysis and make apples-to-apples comparisons across deals.Third, as you’re building your team, focus on finding partners who align with your goals and strengths.

My goal is to build a sustainable rental portfolio that provides great housing for tenants and solid financial returns.I’m looking for advice, insights, and connections from experienced investors or local professionals.