If it were me and I never had a home for the new scope that is causing the issueI'd sell it or trade it for one without the side focus.

The other option is scope swapping within your safe. Is the other 5x20 nikon a SF model? Is the other tikka a wsm or just a 270win? It maybe possible to swap these 2.

I am not much help in regards to which big game high mag variable you should get, b/c the most mag I have is 10x on my big game rifles.