I thought that we Pasken that the very fact that it was mounted by a bull Pasels it (“Alal Aleha Ol” – see second Perek of Parah), and the birth – or for that matter, the pregnancy – merely proves what had happened beforehand to Pasel it. Either way, why wasn’t it kept separate from bulls? This doesn’t seem to be rocket science.
I’m still not sure what all the hype was about. Look up “highland cattle” for thousands of red heifers. The supposed parah adumah near Lakewood didn’t look any different than regular highland cattle specimens; though it is mixed with another breed – Angus, if I recall.
OK, so I looked at pictures of highland cattle. If you tie back the hair in their faces so they can see where they’re going, would the string passul her?