A thick, meaty wad of used twenties, heavy in my hot little hand and stinking of sin.

The multiple heating systems and hot water tanks don't make sense. 

Answer will depend on the market and on how hot the market happens to be at the time. 

Is this the typical interaction model with general contractors?

Depending on the property configuration, I would take two different courses:  A) reconfigure the utilities - relocate hot water heater, shutoffs, etc. to be under your control or B) Negotiate with the adjoining owners to get a key. 

There is a brand new development downtown called South Main that will bring 314 new units online this spring with rents starting at ~$1,400 and amenities that include high-end finishes, a year-round pool, hot tub, community BBQ, covered gazebo, private gym, lounge, pet spa, gear workshop, and co-working space.

So that means you'll need to be under contract (easier said than done) by the beginning of June or late May (always a hot time for the real estate market), which means you'll need to start shopping in April and be ready to pull the trigger in May. 

From what I can tell, hot is on full-open and cold is completely shut.The middle line runs to......the toilet.Yep.

Maybe shadow someone else doing flips in your area to get a handle on what they do so you can recreate parts of their business model.

https://www.adventuresincre.com/single-tenant-nnn-lease-valuation-model/Great writeup