Also, I believe I meet those other requirements, such as purchased by an LLC, keeping of clearly separate records, etc.

Keep the "7 degree of separation". feel free to message me with any questions!

You will always want to separate lot rent income and associated expenses from additional rental income and associated POH expenses.

This allows you to both operate as separate entities without the unnecessary entanglements that come from partnerships.  

I don't have a separate bank account for investments. 

I did already include property taxes separately so including them would be “double dipping”.

If not, no hard feelings and everyone goes their separate ways.

On a six unit apartment you will need to go for commercial financing but with the SFRs you could go for residential lending or you could look for a portfolio lender who would lend on 6 separate properties inside of a single loan.

If you are not helping to qualify, you typically wouldn't be on title, so you'd have a separate legal joint venture agreement showing your interest in the property and the splits. 

Or would I then have to file a separate US return for the business?