4 . Treatment
There are so many wood treatments out there it can get confusing. But the main things you are looking for on the tin are “preservative”, “mould resistant ” and one that will keep the colour that you want. I don’t personally like varnish as they tend to flake and peel after a few years. I prefer the various oils that are out there such as decking oil as these soak right into the wood and have built in mould inhibitors and preservatives. When I sell a carving I will tell you what I have used to treat it and when you should reapply the coating. But as a guide I would say to recoat whenever the carving starts to look dry, or at least twice a year in Spring and Autumn.
5 . Mould
Living in such a damp climate as we do, mould could be an issue. But by keeping your carving off the bare ground and regular recoating of the wood treatment with built in mould inhibitors, then it should stay mould free. If however you do get mould developing then this can be treated with a solution of bleach and water. Scrub the area with the solution and allow to dry before recoating with the oil.
One more option would be to have the whole carving pressure treated. This is the same process that sawmills use to treat fence posts, it will give the piece a 20 year life. I would only do this with woods that are not so naturally durable . The only drawback is that the wood will turn a light green colour, but this can be easily stained over.