So glad you asked this question HB. I was just wondering about blanketing a wet horse the other day, though none of mine were wet or shivering, just a random thought on my part.
Here in Maine, actually right now, it is very cold. There is also a snow falling and the backs of the horses are crusted in snow. While neither is shivering, it is good to know that if this should happen, I'll know what to do now.
The only thing I can contribute, since I do have limited knowledge about blanketing, and don't have to very often, is that I offer extra hay to them when the temps and wind is brutal.
If your older horse is on pasture, well then it can graze for the extra it would need to stay warmer. If not, then toss it a few extra flakes in addition to the covers.
Sounds like things are under control, and you're a caring horse owner. Hope that crazy weather there starts to subside.
Be the kind of woman that when your feet hit the floor each morning the devil says, "Oh crap, she's up!".