A:AnswerAs per screenshot from Dell this DOES NOT void the warranty so Best Buy is incorrect. Only if you damage the part while replacing the FAULTY thermal pads or paste will it void the warranty. Be careful and do a good job and you have nothing to worry.
A:Answerwhile it is correct that the oculus quest is a stand alone VR headset it also includes a feature in which it can be connected to a PC and operated as a PCVR headset as long as that PC meets the requirements to run a VR headset. So the answer would be yes