If western europe was looking to square up to Russia to protect an eastern europe buffer zone, they could have used the Budapest memorandum on security assurances as a pretext to offer military assistance the moment Russian troops started crossing the border.

One could reasonably argue that Ukraine simply wasn't part of the EU, and if Russia threatened Poland it would be a different matter. That it is the EU membership, not the buffer zone, that is important.

Another possible interpretation is EU would have wanted to intervene, but its current political structure proved too deliberative to react before it was too late. That would imply an EU army would also need political reform.

No idea, but the point I was responding that Europeans would never let Russia dominate a border country, and the Ukraine seems like a counter point.