Three thousand lingots!!! lol.
I actually don't need a mnemonic for "caballo" since it's the same word in my language (even "cebolla", too, at least, practically), but I really loved your cab --> transport--> CABallo thing. Very simple but very effective, I would think, to those who need it. I also have always looked out for your comments.
Go by sounds, they sound really different but look alike.
Cebolla, to an english it would read like sibowla. What goes in a bowl of food, onions. It also sounds like Si-voy-ya
Caballo, sounds way different, although to me they look the so similar in spelling. Caballo reads like cab-allo, or hello cab. Imagine a horse drawn carriage or "cab". It sounds like ca-vay-yo.
"You do not touch" means "Tú no tocas" while
"You cannot touch" would mean "Tú no puedes tocar".
"Do" in the 1st sentence acts as a helping verb to the verb 'touch'.
On the other hand, "can" in the 2nd sentence is an additional verb to the sentence. You would have to add the verb "poder" (meaning "to be able to" or "can") to the sentence.
That is why the "Tú no tocas" cannot mean "You cannot touch".