As a loose rule of thumb, I have heard a primary residence requires the owner lives there the majority of the time each year (i.e. more than6 months), but a lender should be able to confirm the specific criteria.

If your willing to do so, you can live in the casita and then rent out the main home and have your tenant pay the majority/all of the mortgage (depending on what type of deal you find obviously). 

My biggest challenge right now is finding a property management company that offers rent-by-the-room leasing services.

New construction in a saturated market isn’t always the best play if you’re looking for steady returns.A place like Rochester has way better fundamentals—lower buy-in, strong rent-to-price ratios, and stable demand from universities, hospitals, and major employers.

One major things to consider that many newer investors miss with 5+ units, is that if you intend to hold, the property will need to be at least 75% occupied and stabilized before you will be able to find reasonable financing terms. 

I would also recommend getting a few more bids from reputable foundation companies to ensure you are getting the best quote.

What about if you set up a maintenance company that maintains all of the properties, then you have a real estate attorney prepare all of the lease agreements.

Yes, some immigrants send remittances to their families, but the majority of their earnings are spent locally—on rent, groceries, gas, and other essentials—directly supporting the U.S. economy.

If you are open to value add strategy, small multis have potential for not only traditional forced appreciation but also the ability to see major rent increases.  

Here's an article with things to consider when selecting a cost segregation company.