Obviously with the price I am discussing 2-4plex, duplex being most common by far.

There is another rule that is not commonly used by investors.

Generally tenants pay all utilities, but its very common for landlord to pay water around me.

Is it okay or at all common to use one realtor for buyer and seller?

., 1. it is common to be limited to 3 draws.

In a large apartment building, yes, 30 or even 60 days is common but not on a SFR.You stated that you disclosed to your sellers that you would be or maybe assigning to another buyer.

What are the top pitfalls and how to navigate them? 

The most common way for the city to find that out is information on where mail goes.

Here are some examples of where "substantially the same" might come into play: You identify an undivided 2.05% tenant in common interest in a property but you actually end up acquiring a 3.05% interest in the property. 

If you do decide to sell, then strongly consider working with an real estate broker who has experience and a track record with multi-units to avoid numerous pitfalls.