What is the minimum mass of an ROV that could be precisely maneuvered be two T-100s?
This would depend on the vehicle shape, density, and dynamics. A better question to ask is how much thrust does the T-100 produce? The answer can be found in the product specifications.
Why would a T-100 not respond until a significant amount of throttle was given to it?
This is not a property of the motor, but the electronic speed controller. Some electronic speed controllers have a dead band where they will not produce any output. This will also depend on the power being sent to the thruster for example, if your supply is a AA battery, that might not provide enough power to produce the torque necessary to turn the motor.
Should the t-100s be able to operate low RPMs like 60 RPM?
Again, this is dependent on the electronic speed controller. Usually speeds this low are only possible with ESCs that operate with Field Oriented Control (FOC).
What would cause a T-100 to surge to high speeds and reverse directions when not being given any throttle for a minute then stop and not respond to anything?
This sounds like you have spurious/unwanted signals going to the ESC. It is very unlikely that this would happen otherwise. You should verify your software, and physical connections/continuity of wiring. An oscilloscope or logic analyzer will reveal the issue immediately.
A PWM signal out of range?
Too low of a voltage on the signal line?
Too fast of a refresh rate on the PWM signal?
Yes, yes and yes.