"Is there a way to actually measure rocker on a boat?" lots of ways, but no standard for the industry

"Is rocker the same as the shear line at a given depth?" -

no - see below
---Shear-line: top edge extending around the canoe hull from bow to stern. Skid plate: A piece of thick Kevlar® that is glued to the bottom ends of a canoe. Prevents abrasion of the skin of the canoe. Stem: The most extended part of the bow or stern, ascending from the keel to the deck.---

"If I do buy a Magic, what will I be gaining over the Indy other than 10 to 12 lbs lighter?"

You should definitely gain some speed - Magic will have a higher length to width ratio, so faster for that reason - but simply pushing 15 lbs less weight should gain you a bit more speed itself.

I have the Kevlight version of  the Merlin II that is the black/yellow Kevlar weave (no carbon) - I wet foot and try to be as gentle as I can, but it has held up very well even hitting the occasional rock - If the object of your new canoe is to get a lighter boat, I would get the lightest version available - and that likely includes the aluminum gunnels rather than wood - but to each their own.
I suspect that you might lose a little bit of stability in magic vs Indy - but maybe not - best to test drive anything you are buying, with and without a tripping load