What is "au sec"?

I would read that to mean reducing to the point of it being syrupy. You can go with moderate heat until you sense it thickening, then you probably should turn it down to be very careful not to burn it. It's the kind of thing that will seem a lot easier the second time you do it with the same recipe.

Use your smallest saucepan.

You probably won't need a whisk, but keep one handy just in case. A wooden spoon is probably your best implement.

If at any point you feel like it's getting away from you (getting lumpy or otherwise weird), remove it from the heat and whisk it a bit.

Usually recipes will say to reduce by half or two thirds, or it will say until you have a certain amount left. Since this one doesn't, go with your gut. You're done when it has thickened and will nicely flavor a pint of crème fraîche without making it watery.

That term is used in recipes for Beurre Blanc; look at this recipe from Epicurious. It's the same idea.

Let us know how it goes. It's an interesting question.

You are reducing water with peel and vinegar and juice, so nothing can coagulate or curdle, and you can make it easily and quickly.

Use a big pan with a lot of evaporating surface over high heat and stir constantly until you have just a little bit left, you are only doing this to concentrate the taste and not to thin out the cream too much.

You wont get more taste out of lemon peel by boiling it a long time. If anything, it is likely to make them bitter.