All good questions, and no definitive answers.

Best layer count for Chatoyance? That's a mixed bag because chatoyance is the way light reflects across a surface and produces an optical phenomenon that looks like movement. It's not for certain that layer count improves the effect or decreases it in a proportional ratio. The same is true for multiple types of steel.

Some folks like to throw in a few layers of pure nickel sheet in their billets because the nickel really reflects a lot of light and produces a very high contrast and some very shiny spots (even more so than 15N20 or L6)Nickel is not hardenable, so don't get carried away with it.

My personal opinion for a ladder pattern is based on what I have seen, more than what I have made, because I've seen a lot more ladder pattern blades than I have made, or will ever make. I would suggest something in the 250-400 layer range and a few (4-6) layers of pure nickel in the initial billet. YMMV depending on the thickness of the nickel relative to the other steels you use.

I do not know how much experience you have making pattern welded steel, but Knife & Gun Finishing Supplies sells 1080, 15N20, and pure nickel in compatible sizes for Damascus making. This is a classic combo and should be fairly easy to work.