Refer to the manufacturer of the fixture for their recommendations regarding your questions. Do not believe rules of thumb you read online.

Sorry but, your designer doesn't know jack about lighting art. You don't decide on the height of art lighting independent of the piece of art. Find a piece of art you love. Hang it on the wall at the height it looks best and is easiest to see. Then place your picture light above so it illuminates the artwork.

I found a lighting designer on Houzz. She was fantastic in lighting our artwork among other things. Here are some pics of lighting artwork from her portfolio.

Midtown East Lighting Design · More Info

Art Lighting · More Info

Philadelphia Dining Room Lighting Design · More Info

Read the thread about rules! No, not all art needs to be at 60", or more accurately, the middle of the art piece should be at eye-level ... but they never say whose eye level, the wife at 5 ft tall or the husband at 6'3" tall or the cousin in the NBA at 7 ft tall.

I have art at all different heights, depending on the art and the actual space where it hangs. An old friend, who had collected art all her life and worked in the art business, had every wall in her house covered in art, from floor to ceilings! My walls always felt bare after visiting her!

I agree with Chispa. I have 10' ceilings and art all over the place. I have never pulled out a measuring tape to guide where the 60" fits in my space. Hanging art is an art. Sometimes huge pieces can create huge impact. Small pieces in the right space can do the same. I have more than a handful of pieces hanging just above the baseboards. Following all the rules will lead to a very boring space.

The "rule" is that the center of the art is to be hung 57" from the floor. I find this too low so I use 60". The location of the light is going to depend on the size of the art piece unless the light is on the ceiling as it is in museums and galleries. Consensus: Picture light is 1/3rd to 2/3rds the width of the picture, centered on the picture and 6-7" above it.

I would have them leave the wiring in the wall with some extra wire for the time being, and when you find a piece of art that you like, they can then install the art lighting in the appropriate location.

"They have spec'd art lights for these locations to be 9' off the ground..."

You could place a recessed wall outlet, say, 60-66" above the floor where you plan to hang the art. When the time comes, hang your art, attach a picture light above it, and plug the picture light into the recessed outlet behind the art.