The downside of trying to get a mortgage after you have a rental is that you can't walk into any bank because they do not understand how to calculate DTI correctly. 

In my experience, they use their own judgement of what is right and fair over legal correctness in many cases.

Yes, what you wrote is correct.

It could be a great resource to make sure you’re evaluating the property correctly!

Creating these complex structures from what I have experienced only costs me time and money and potential issues down the line to make sure documents are signed by the correct people and entities etc.

@Matthew McGillJust to confirm .... you will live in one of the units of the duplex, correct? 

Am I correct in this assumption?

If I'm wrong, please, somebody correct me!

Generally speaking, the responsibility for repairs like this depends on the lease terms and local landlord-tenant laws.If it was a malfunction with the lock (wear and tear), it could be considered the landlord's responsibility to cover the cost, since maintaining the unit in a habitable condition is part of our duty.

@Mike S. is absolutely correct.