Yew provides the best wood for the longbow and during the conflicts with France this was a massively prized commodity. Any yew tree that survived that period must have been held very special.

You mention the theory that yews were planted in church yards to keep them away from cattle. Why plant them at all? There must be a reason why they were planted or why they were left in place if they are older than the church. If they do 'keep away the devil' and the churches were built next to them then site was chosen because of the tree. Obviously, you can't rule out an earlier demolished or wooden church from being on the site before the present one. Only if the yew is over 1800 years old can you say that its 'sacredness' predates the church - otherwise it could have been planted by the first monks to settle there to 'keep away the devil'. But then where did they get that idea from?

Lots of questions indeed.