Yes, you can definitely ovulate at different times during your cycle - and as the PP mentioned, your luteal phase will generally be the same length. It would be the follicular phase that fluctuates. If your cycle is the same length every time, then you are probably ovulating at the same time every month. If your cycle changes, most likely, you ovulated earlier or later, but the time from O to AF will be the same.
My cycle is anywhere from 28-45 days long, but my LP is always 12 days. As far as the estrogen withdrawal bleeding, I can't help you out. As far as I know, estrogen surges right before ovulation, and causes the EWCM that we all look for, and then it drops again, and progesterone takes over. (Maybe that's where mid-cycle spotting comes in for some women????) Progesterone causes the elevated temps that those who chart note as a sign of ovulation. When the progesterone drops, your temp goes down, and AF arrives. (Or, if you conceive, the progesterone continues to climb, and tells your body you are pregnant.)
That is sort of how it goes to the best of my knowledge. If I've got this wrong, someone please feel free to correct me!
HTH!