Performed extensive (mostly) cosmetic updates to flooring, mechanical systems, appliances, reworked kitchen to create a more modern open floor plan (removed walls) w/ island, and bathroom.

Ideally in a finish that matches pulls.If the budget is higher, I would rework the layout, as others mentioned. 

We are leaning towards going for it since buyers look for that but reworking the existing cabinetry to save on that end.Thanks a million for your advice, all!

Rework is expensive.Depending on how much the other owner is willing to give, first ask if she is open to selling you the land under the carriage house. 

This re-work cost seems like it should be eaten by the contractor, but contractors understand well the situation that homeowners are in here and can easily create a nuanced "pay for this knee-cap insurance or you might find your knee-caps busted up" scenario.I may be misunderstanding the situation with the electrician, but if this is the same contractor that didn't do it right the first time, then ^^ holds 

Contact him and help him understand how however you are involved in RE works.

I have single-family rentals and I would say that the biggest missed opportunity I see from other landlords is general laziness - not painting worn paneling; refusal to re-work odd spaces or configurations; not putting in a dishwasher; etc. all things that in my area limit the desirability or marketability of the property.

Without dimensions, it is hard to say, and it will certainly require reworking plumbing.  

If you are interested, keep reading, I will explain and summarize the best I can how larger RE works within some commercial/industrial/government complexes (cluster of systems/buildings, generally speaking).For more complicated systems, there are software/parameters based on previous research to do a more deep analysis of the maintenance and repairs based on condition. 

You might want to rework your numbers.