Why modulating?
True you can get more efficiency using different approaches, like outdoor reset, for one.
Don't think he wants to redesign the system any more than he has to.
I think we should assume...
a working thermostat.
The system is not air locked.
no windows are knocked out.
Tightening up a room for heat loss is always an option.
Since the first floor is a shorter loop flow should be faster than the other floor.
As you already stated, the second floor always tends to get hotter.
Just briefly...," baseboard will put out pretty similar outputs over a very broad range of flow rates."
Now why would a landlord want to have control of the heat in the tenants apartment, as currently set up?