Would these work to stretch the vamp of a leather shoe? I have a pair of Born shoes that are tight across the top but fit fine in width and length.

These might work for a woman's high heels. They weren't strong or wide enough for my 10.5 2e size men's shoes.

Will this work on high heel boots?

Doubtful, I couldn't get them to work on my low profile boots... I tossed them 5 minutes after I received them.

What are these made of?

Hard plastic ends with a springy middle. They worked on my flats and heels.

Are the two sides counter-threaded (so you can put them inside the shoe and twist the turnbuckle to expand)?

I simply opened it as wide as it will go and pushed it back in the toe of the shoe and have left it there. I still have not worn the shoes but only left foot is bigger and needed stretched. Yes, they are cheaply made but they didn't cost a lot and if they do the job then I'm happy.