Shopping using the max of all max prices is not going to happen, but leaving open for the possibility that shopping just uses the first one that applies.
Why are itemIds important exactly?
You can already do this in-game. What would be so much more beneficial about a web-based tool?
as you have said there are other addons out there that already let you do this so i dont see any value in us adding this
I was able to search for it just fine: https://www.tradeskillmaster.com/items/72146
What mount don't you see?
You can already do this for item pages, just click the "Link this Page" button at the top.