I wonder how "the worker is [not] benefitting from their physical location" would be interpreted in practice? Surely if the worker is in the USA they ALWAYS have SOME reason why they are there--and thus they could always be said to be "benefitting" from the physical location. It might not always be a benefit to the Canadian firm, though.
The benefit I am referring to is the "unique" benefit from being in US to do this work; ie. the person could not do this work without being in US. I gave the example of doing research on a particular US market, or studying a particular 'thing' that is physically located in US. Doing soild collection study in AZ for example. This could not be done anywhere else than physically in US, thus this would not be incidental to their main purpose for entering US, the worker should get a
TN or B1. Remember the worker can be self-employed, so when I say the worker is benefitting, I'm referring to him as either the firm itself or the employee.
As to what constitutes US presence for the firm. that is a little more tricky, as you point out.
As our posters university pointed out, F1 has restrictions; B1 also has restruictions, but they may not be the same. The safest course is to do what the school suggest: go to canada to work.
I'll answer the tax issues on serbinski, but as I pointed out, you will not be considered resident of US in all this -- because a student on F1 cannot be a US tax resident for a few years, so it will be like you are going to school in BC or ontario. You wioll only report US income in US, and ALL income in canada.