My point is that if you sincerely try to think through the trial and error required to get from one type of heart to the one you think it evolved to, you will discover that it is simply impossible. Try it for the task of evolving the mammalian ear from the reptilian. Sincerely, I said.
I am just trying to describe what is possible using my own understanding of biology. Is there any reason to think I’m being insincere in this?
In the case of the heart we know a thickening of the ventricular wall will split it into two ventricles because that is what happens to our own heart during foetal development and results from how and when our genes are expressed. This change prevents oxygenated blood from the lungs mixing with deoxygenated blood returning from the rest of the body, which makes it more efficient, especially as we need a constant supply of oxygen as endotherms to maintain our internal body temperature. This is not as important for frogs and reptiles as they use the environment to control body temperature, but even a partial change in the shape of the ventricle wall, as in the 3.5 chambers mentioned by dwise1, will help to control the flow and reduce this mixing.
As you mentioned, the formation of the ear is another example of this principle. During foetal development we see formation of the Meckel's cartilage. Most of this cartilage becomes absorbed into our lower jaw, but part of it splits off and migrates to form bones of the middle ear. Again this is controlled by expression of our genes, and some of the intermediary steps are reflected in some reptile fossils, so we know it is possible.
The point is that changes is gene expression during development can affect an individuals physiology, which would be described as variation within the population, but over many generations and favourable selection this variation can come to dominate the population. So a 3-chamber heart can become 4-chambered, or to give another example, a reptiles jaw can become broader and the musculature of their intestine can be modified. This is why I tried the analogy of origami, but I’m guessing it wasn’t as helpful as I hoped. Such is the way with analogies.