Not fully understanding how it works, if I have a PAS sensor and a throttle attached at the same time (not on the some connections), can I vary the power to the PAS sensor via the throttle or would it just be full power continuous?
Depends on your controller's design. If it's manual doesn't tell you, you'll have to experiment to find out...but:
Almost all of them use the throttle as variable assist control (in one of a few ways), but use PAS only as an on/off (pedalling or not pedalling, regardless of cadence) control for the chosen assist level. So if your controller uses say, 3 assist levels, one set to 5mph, one to 10mph, and one to 15mph, then if you set assist level 2 and pedal, at all, it will simply try to go 10mph. If you slow your pedalling drastically, it still tries to go 10mph. If you stop pedalling, then after a few seconds, it stops trying to go at all.
The throttle would still give you variable control.
Some controllers vary between zero and the max for that assist level, some vary between zero and the max the bike is capable of under present conditions regardless of assist level.
Some controllers only allow throttle in certain assist modes, some in all of them.
Some controllers have no throttle input (and some have no PAS input).
Some controllers require you to be pedalling for the throttle to even be allowed to work. (some of these only do this below a certain speed, or above it). These are just legalization functions for certain areas around the world to comply with local regulations; there is no practical reason to require them.
If you need variable control the best kind of controller to get is a throttle-only type, and then use one of the things I previously posted to generate a throttle signal from your PAS unit's cadence output. (or a torque sensor if you prefer that, but it's a lot more complicated to setup and tune to get exactly your desired behavior, and has some potential gotchas you have to create or enable/setup safety features for unless you can be certain to avoid them).
Or something like the KT or Lishui controllers flashed with the Open Source Firmware developed in various threads here on ES, allowing you to use them with variable PAS control or throttle...keeping in mind that you'll still have to setup all the menus and settings for them to perform the way you want, pretty much like setting up a Cycle Analyst (probably a bit less complicated, though).