Fertilization would depend on what your soil is lacking. Is it worth it for you to get a
soil test done? Have you at least done a pH test?
If not, then I'd just spread a light covering of some kind of manure around the tree on the soil surface. Since you've already spread
compost and
mulch, I'd spread the manure in a circle around the existing mulch ring, about 1/2 to 1 foot wide, then cover it with wood chips or some other mulch. You may want to sprinkle a light dusting of
wood ash around the tree as well.
To help mitigate the cold...
Place as many rocks around your trees as you can. The more, the better. To be safe though, I wouldn't put rocks within 1-2 foot of the trunks.
Water...if you can; create ponds, even small ones near your trees. Generally, placing ponds to the south of the trees is best because it'll reflect light towards them.