I like both of your ideas so I would look for the single where an ADU is permitted or one with a dump ADU to renovate (that's best) and balance that against the purchase of a 2-4 unit.

Registration is only permitted for legal entities, partnerships and natural persons who are 18 years or older.

GC never pulled any city permits yet charged me (didn't find this out until the job was finished). 

Duplex have different zoning that may permit additional units.

And I have below clause in the leaseRental Application: Only those people indicated on Tenant’s rental application are permitted to reside at the Premises, with the exception of any minor children born to, or adopted by, Tenant.

Built without any permits.  

I'm an Architect and MRED Grad and have entitled and permitted over 200 units across Bay Area and Norther California, ask me anything about SB9 or ADU design / permitting

If your leasing application process is thorough and well thought out, and if your lease agreement has been well drafted by your local attorney and if it includes clauses that permit you/your property manager enough latitude to maneuver and anticipate undesirable tenant behaviors, both actions and inactions, and you have operational processes which align with enforcement of your lease, then you've done all you can do to reduce this inherent risk of an undesirable tenant.  

Partnering with local experts who understand zoning, permitting, and market nuances can help you avoid costly mistakes and maximize returns.If you're in a high-cost area, out-of-state investing can provide access to markets with better entry points and higher ROI potential.

You may could give them a license to use the land, provide a temporary or revocable easement or otherwise permit the use in some way.