How to tell if the amplifier is voltage-to-voltage, voltage-to-current, etc

I'm more of a hardware person rather than software, so I immediately look for hardware solutions.

My approach to this problem would be to have a single AC signal source followed by several digital pots. Each of those digital pots is followed by an appropriate buffer or power amplifier that then feeds one actuator.

You can generate the signal either using a single PWM output from the controller (appropriately filtered) or with any of the very many analog hardware techniques that are available.

You will need to decide how much amplitude resolution that you need. You should also find out how much amplitude variation you need - does it have to range all the way from zero to maximum amplitude or does it need to vary only a certain percentage of the range?

Both of those questions will determine how many steps your digital pot needs to have.

If it turns out that you need extremely fine resolution, you may be better off using a multiplying D/A converter instead of digital pots. Such converters can act as digital attenuators and they are available in bit sizes up to or exceeding 16 bits. You would feed your AC signal source into the D/A reference and the code word that you send into the converter determines the output amplitude.

Both of these techniques can be fairly inexpensive.

You are most likely experiencing the effects of current in the return path of your switcher. You can easily check this by measuring the ground with your scope on the ADC ground and on the ground near your sense circuits on the power board (without moving the scope ground). You will likely see the same 100 mV noise between the grounds.

When mixing power with signal, you will have keep in mind that the current return paths back to the battery and the load are not at the same instantaneous voltage at various points along the paths, due to the inductance and the resistance of the path. The paths for signal voltage references should be separate, connected at a single point, and should not have any current (don't share paths or use a ground plane that carries current). If you do have a ground plane, keep your reference signal path separated from it; even if it is on another layer, if you run a signal over a current-carrying ground plane, you can capacitively couple the switching noise.

Hope this helps.