Does a stim patch remove (temporarily) stun damage from fading and drain?
YesNo. You can not heal fading or drain in any other way than resting.
The wording of Stim patch is that it remove (permanently heal) the damage.
When the duration is up the patient take (new) stun damage.
What happens when an effect removes a number of stun (or physical) damage greater than the amount on a condition monitor?
Nothing.
(It removes a number of boxes of Stun damage (not physical) equal to its Rating. The effect of removing a number of boxes of Stun damage last for (Rating x 10) minutes).
Does a stim patch rating 3 used on a character with 1 stun damage still cause 4 stun damage after 30 minutes?
Yes.
(So in this case it would have been better to use a stim patch rating 1 or wait until you suffer more stun damage before you use the stim patch rating 3)
Can you have additional SINs loaded (but not active) on your comlink and switch between them like programs or do you need a separate comlink for each SIN?
You can have multiple (often fake) SINs
stored on a commlink and switch between them at will. You can only have
one SIN loaded (or currently active - same thing) per commlink.
At a security checkpoint or when you legally buy an item you will only display one SIN; the loaded or currently active SIN.