I have ivy growing through my hedge and no it doesn't but gives the hedge a thickness.

Highly unlikely, I would say.
Laurel is a tough old thing.
I have been thinking about growing Clematis Blue Angel through the spotty variety I have.

In my experience ivy kills. It doesn't do it on purpose but it can smother. I've removed it from a cherry tree and many of the libs were lifeless. I personally don't like ivy and would not grow it with anything but for those who do I assume you choose a smaller variety. It also gives a resting place for slugs and snails.