---

The fact that you can open the crack by adding weight to the stern definitely indicates that the boat is flexing at that point. Assuming you now have the boat on a trailer, does the crack close if you jack up your bow, or stern. I would carefully observe what is closing as the jacking is in progress.

If so then the problem is not in sidewall flex but would seem serious. If serious why is there no indication of a crack on deck.

The foam cracking is no big deal as far as structure except it indicates the glass is failing, and that is structural.
The foam is only structural in that it attaches glass to glass like a surfboard. Glass fails and the foam is just flotation.