No, the conception and Incarnation were supernatural, miraculous events. The reuslting pregnancy was a miracle. It does not follow any logic to claim that His birth must have been "natural." At which point did His existence in Mary cease to be a miracle and just an "ordinary" event to warrant an ordinary" Birth?
Yes, it does. The vigin conception Is what makes it a virgin birth. The virgin conception is given in scripture. without any such similar evidence, the birth as a "light through a window" is pure invention.
"At which point did His existence in Mary cease to be a miracle and just an "ordinary" event to warrant an ordinary" Birth?"
That would be when a viable diploid egg was present.