What you say is true but I don't see why people make assumption 1. Does anomalous thrust tell you it can accelerate linearly to power?

The thrust doesn't need to be a linear function of input power for this argument to work. As long as you can get some constant thrust for some constant power input, kinetic energy will eventually exceed the energy put into the system.

True. But how do we know thrust doesn't go down as velocity increases? I feel like we don't have that information yet.

Maybe, but velocity is relative. So that would mean that the drive is pushing against something which has its own rest frame, relative to which velocity can be measured.

It's not clear what that rest frame would be. Virtual particles in the quantum vacuum are often mentioned together with the emDrive, but those particles are thought to be frameless.

I'll admit that it's not entirely implausible to have a special rest frame like that. We already know that the cosmic microwave background has a rest frame, and this is something that you would start to feel in the form of frontal radiation pressure as you gain speed relative to the CMB.