Knowing that they are planning to cheat the IRS makes you an accessory to the crime.

Pay attention to what Jimmy V said, because it took me about 6 months to have the property re-zoned as a SFH with an accessory unit (no separate metering) but it was all worth it as the 5 bedroom unit rents for $3,450 a month and the accessory unit (basement) pulls in $1,500 a month.

2) Sounds like there are 2 legal, conforming units and then a grand-fathered/accessory garden unit where you would live?  

Some towns allow such apartments as an accessory use.

The concept of an ADU is that it is an accessory dwelling on the Same lot as an existing home. 

Most people would rather live in a detached unit of a multifamily than an attached unit...I currently live in one of my multifamily properties that has a shared wall which is perfectly fine with me but currently looking for the next place and ideally it would be detached units for more privacy...Duplexes  generally tend to be smaller here in San Diego (2 bed/ 1 bath units are most common) and those with families and kids tend to want atleast a 3bed/2bath which maxes a single family home with an Accessory Dwelling Unit (ADU) more attractive.

The term “transient accommodation uses” includes but is not limited to hotels, motels, recreationalvehicle parks, tourist lodging facilities, resort condominiums, resort dwellings, vacation resorts, and dwelling units occupied or available foroccupancy on an interval ownership or “time share” basis, when any of the above are made available for occupancy more than three times in anyconsecutive 365‐day period and the right of occupancy is for a term less than monthly.The term “transient accommodation uses” does not include any of the following uses if such use otherwise complies with the applicablerequirements of the City and is licensed by the State of Florida, if such licensing is required by law: bed and breakfast homes, community residentialhomes, nursing homes, rehabilitation facilities for persons with drug, alcohol, or physical impairments, respite care facilities for persons withterminal illnesses and their families, short‐term/emergency housing or long term housing where allowed by this chapter, and child foster homes.The term “transient accommodation uses” does not include a guest house dwelling, when one or both of the sleeping rooms are located as apermitted accessory use within and incidental to the primary residential structure and the primary residential structure is owned by a naturalperson and occupied by the owner.

It helps if you are willing to consider properties that are not zoned as duplexes but have a "mother in law" unit or accessory dwelling space.You really don't want to hold a live-in property in an LLC, btw.

Adding Accessory Dwelling Unit with attached garage and workshop.

Not currently easily atleast but they are in talks about changing the zoning laws to where you can build/legalize accessory units soon.