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

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

Too many investors get their ideas and emotions so mixed up in the process of buying and selling, they end up failing.

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?

I'm afraid to look seriously do to my emotions getting involved and not allow me to look at a deal with clarity. 

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

With all this stress they've caused, maybe you need an emotional support tiger.