Maybe I don't understand "relative" in this case. Here is how I am meaning it.
The "main" bar is whichever is shown at the bottom. They key bindings for these start at the left and move right. If the user rotates the inventory bars, all items shift through and a new set is visible on the main. The key bindings are for the slots, and not the spells.
For the key binds I was suggesting, this would work just like this, except working on the second through tenth bars.
I understand what you mean by relative, and that's also how I meant it.
Specific binds for every spell would be completely untenable, with updates and usability issues.
That's my thought also, and why I said "key spells." Basically, what I was thinking was this:
1. Have a keybind for "wooden platform" that is default unbound. If you bind that to, say, G, then you know you can always hit G to trigger wooden platforms no matter where they are in your inventory.
2. Do the same thing for the shields, the light spells, maybe teleport... and that's about it.
Odds are, most players would not use any of that. But if ball of light is central to how you play, then you'd map that and possibly wooden platforms to... whatever you want. Then you can just throw them anywhere in your inventory and not worry about their position, relative or otherwise, and always know you can trigger them. And meanwhile you can scroll through differently themed bars to get different combat spells at any given time.
It's not a replacement for what you were describing, but it's a far simpler way of hitting the most common cases.