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?

As soon as that renovated property was rented I purchased a 3rd duplex witch didn’t have separate utilities.

Etc, etc, etcYou should at least find out who he is and run a back ground check.In addition, you would have to evict him separately from you tenant if it ever came to that.

Keep your books straight from day 1 - saving receipts, opening up new accounts per property, getting a business cc to keep costs separate.

The LLC, being a legal entity, has to be operated separate from yourself and not as your alter-ego.