Why does boiling cream have such a drastic effect on my salted caramel?
You don't tell us neither the ratio of double cream to caramel, nor the time you heat the completed mix, so this is just a guess. But it sounds logical that your problem is evaporation. 

Double cream consists mainly of fat and water. I don't remember the exact percentages, but more than half of it is water. So, if you heat cream, part of it evaporates during the heating, and it continues evaporating after being added to the pan with bubbling caramel. But if you use cold cream, its water hasn't evaporated during a heating phase, and if you add a large amount of cream to a hot caramel mixture, the whole mixture cools considerably. If you don't heat it and let it simmer for long enough afterwards, the water content of the cream continues thinning the spread. You end up with a runny sauce instead of a sticky spread. 

Instead of pre-boiling, you should be able to just simmer for longer time after adding the cream, if you find it more convenient. But you'll have to try it a few times until you have found the cooking time which gives you optimal consistency.