Is the prayer valid when one places paper tissues after Istinja' due to light urinary incontinence, then removes them, performs ablution, and prays?

According to the majority of scholars, you must perform istinja' again if you are certain that something, like urine, has exited. However, if you are not certain, then istinja' is not obligatory. The opinion of the majority is more cautious and absolves one's responsibility.

You must ignore obsessive thoughts (waswas) and should not rule that something has exited unless you have a certainty that you could swear by. As for your past prayers, you are not required to repeat them.