What is the ruling on interrupting prayer due to the obsessive thought of intending to interrupt it, and what is the ruling on feigning something to a friend to cover that up, and is this action of mine considered shirk (polytheism)?

One must be wary of whisperings (waswas) and ignore them. Hesitation in the intention to terminate the prayer does not invalidate it; this is a strong opinion favored by Ibn 'Uthaymeen. As for prostrating with the intention of relieving discomfort, this is not legislated, and nothing led you to this except yielding to the whisperings. You must abandon these whisperings and not yield to them.