If you have in mind some level of humidity that those tools need to be stored at, you might want to get a hygrometer to see where it CURRENTLY is (assuming this is presently the "acceptable" range), then see what reducing the temps will do. FWIW: I don't have very fancy tools, but I do not have to heat my garage to keep my tools in good shape. Now, I don't leave my tools on the ground, and it's warmer here than it is where you live, but I doubt they need to be anywhere near as warm as you think they do. I did some quick Googling, and there seems to be a consensus that temperature CHANGE is a bigger issue than humidity levels; you don't want more than a 3°C swing, or more than 5% humidity change in a day. If accurate, you could keep the garage at 2°C and it should be fine, but I'd expect your hubby's tool manuals would have recommendations.
Otherwise, @Quentin5 is probably correct on all fronts; you need someone in there to get everything working properly, because you definitely have some issues that have probably been around for a long time (or due to you using your equipment improperly), and you'll not plausibly be able to resolve them yourself (re-balancing your HVAC system might seem easy, but ensuring you don't impact your furnace's required flow rates is best left to someone knowledgeable). Good news is that, with your high energy consumption, and since nothing seems to be actually broken, you'll probably get a return on your investment pretty quick.
I still do suggest a Kill-A-Watt, because if your home came with appliances, you might want to ensure they're not a big problem.