But my semen still definitely flows through my Urethra when I'm doing that as evidenced by the pre-fluid.
This is not quite right. I think we should distinguish between three different potential allergens: 1) sperm, 2) ejaculate-semen, 3) pre-ejaculate fluid.
So, when you masturbate, while stopping before ejaculation, pre-ejaculate (3) flows through your urethra, but semen (2) does not flow through your urethra.
We know that some people develop a POIS episode just from 3, without the occurrence of 1 or 2. And, for instance, in Demo's case, he still has POIS symptoms after getting rid of 1, while 2 and 3 remain for him.
Thus, even if we also want to think about 'regeneration' theories, we should still be sure to take into account and distinguish the various elements that can potentially be contributing to an auto-immune reaction.