Its very hard to date such artefacts. I would not go to say that there were pre Christian crosses.
Anyhow there have been xtians in this country since late Roman times. Most crosses would be about 600-1200?
Yes, lots of pagan imagery. But I think many folk saw no conflict. Odin, as we know, is a god who sacrifices himself for knowlege for mankind.
(And of corse the old monks would have been classical scholars)
The Sigurd story is popular on the Isle of Man.