Hi there! Nope, you didn't miss a rule :)
The incentives I had in place were the leviathan dissolving after you leave it, and the fact that resting and finding another leviathan requires gold. Your single‑delve high score is also an incentive if you are of a competitive mind hehe. But I understand that going back and forth from mouth and brain would essentially solve this.
I like your hacks in any case. Kudos!