Right now the dwelling is nowhere near what it would be with full power draw, and I'm confident we have enough free amperage to run this thing.

Check to see what counts as an accessory dwelling in Brantford - this would be a triplex in all but name and might be easier permit wise.

Dwelling limit is the total amount they will cover for the property in a single loss (or depending on the wording of your policy within a certain term no matter how many losses you have)FYI a "loss" is a claim.All perils deductible means you have coverage for anything that happens to your property UNLESS it is specifically excluded in your policy. 

Currently the property ( old barbershop that must be converted into a single family dwelling ) is in a high valued market.

While it may cost 400K USD upward to cop a good property in the US, one can purchase land for near $150K and build a 10 room dwelling or a townhouse type apartment complex for around $200-250K USD.

TRUST ME, the girlfriend will admit that he was living there and will likely even allow him back into the property against orders.But even if not, if he was residing there, with landlord permission or not, lease or not, he is a TENANT, or at the very least an authorized occupant, and as such cannot be prevented from entering his primary dwelling...not by the landlord.

When you buy a property for a rental, you will want "Dwelling coverage". 

So he insured this last one as a rental dwelling policy.

The laws for renting out MIL’s or “accessory dwelling units” seem to vary drastically City to city.

it varies widely per state...in florida I am basically only required to provide a dwelling that has heat...AC not even required by law.