Your goals, plans, and execution to date are solid but a household needs both an active income stream AND a solid investment/ savings plan and you are inappropriately skewed 100% to the latter.

Unit A has its own meters and its own breaker panel.Unit B, C, and the common area are on a single meter for water and one for power, with a single breaker panel.My Understanding of Sub-metering Power - An electrician should be able to install sub-meters to measure the separate power usage from Unit B, Unit C, and the common area. 

She said she would gladly pay the extra usage but how can I claim her usage without a fight. 

For water usage there are several areas to look at:1.

Including myself, there are only two of us agents underneath him.

I was planning on having them included in the rental price, but I am worried about over usage.

We obtain "legal usage confirmation letters" or "zoning letters" confirming the legal usage of a property from the city.

Once u have that and your mothers bills with the sudden changes in usage, file a small claims action against the owner of the property. 

@Gita Faust, they may use QBs for their 400 properties, but they are missing out on the usage of true management software.

I'd say to read both books very slowly, but this one most of all - keep in mind that he's boiling down months worth of a class into a book, so to get the best of it, I think you really need to review each point as you go along and extrapolate that into relevant usage.