Durability + texture pack can change models/textures. Outside of that I've seen a video where renaming items would change the model accordingly (a mod, I don't have the link though)
You'll probably have a difficult time finding a good illustrator if it's unpaid work. Potentially if it's a well-known project, but otherwise it'll be hard.
Even though there aren't XP orbs a large quantity that increases the level by more than 1 in one go (/xp can do this) it'll still pose a problem. Imo the difficult part is to know how many, and what levels just got passed (i.e each level gained).
Hm.. The suggestions you've been given are honestly your best bets. Other than that I'd suggest you use a custom model. From what I've understood opaque (I think that's what they're called?) blocks don't cause you to suffocate nor push you away. Glass for example. I know that sucks but it might be a viable solution.
As far as I know you cannot detect for air in a slot. However you could detect if there's a diamond there, when the output is false it means the diamond isn't there. Thus we can ASSUME that there's air in that slot and not another kind of item.
Send schematic please? And just to make sure, the issue is that you want to turn it off and on? (the clock), could you please be more informative. Like, do you need it to only be on when you're walking? etc, please elaborate.
