A couple of things - the screws on the headbadge are often like the very small screws on a typewriter, or firearm. I get better bite on them using a hollow ground, very small screw driver as one would use on a pair of eyeglasses or the sort you get in the precision driver sets for firearms repair. A wedge type or woodworking screw driver usually will not work as well as a very small, hollow ground type (again, think eyeglass repair, firearms repair, etc).
I start by trying to just loosen without heat, oil or anything else. If you can just loosen it the plain way, all the better. If you can't do it with just the correct-fitting screwdriver, then I try a little mechanical help. I take small block of wood or very small mallet, and tap the handle of the driver gently as I try loosening the screw. This is the miniature version of the impact driver. You're just jarring the gunk or corrosion loose. This often will get the screws loose because they're not held in by much, except for years of gunk or modest corrosion. Resist the temptation to strike hard - you don't want to flatten the head tube or ruin the threads. After you succeed with this method a couple times, you basically will have the hang of it.
I only resort to heat after those other means have failed. The reason for this is that working with heat, you start to narrow the margin for error. The gunk holding the screw in place and the paint you want to keep both have a scorch point. You're good as long as the paint's scorch point is higher than the gunk. A soldering iron, as you have found, gives you a little more control than any open flame would.
The screws are generally very soft metal and deform quickly, so don't force anything with these unless you are fine with replacing the screws.
