You are correct, there are limitations due to the fact that there is no straight-forward way to apply discounts on the cart page (indeed the cart and checkout page cannot share enough data).
- As you mentioned, we cannot check whether someone already applied a discount on the checkout page, and then went back to the cart.
- Your second observation is also correct - while we can signal to Shopify that a discount should be applied, we cannot signal to them that we no longer want it to be applied.
Even with these two edge cases in mind, we’ve seen conversion rates jump by two-digit percentages in some cases when using the current solution that we offer. We are still in the process of gathering data, when we are ready we will publish some case studies showing before/after stats.