Definitely, thanks for sharing.
I think the problem with that strategy though is perhaps that it is executed at a level whereby even WooCommerce is unaware of it. What I mean by that is, for the ticket form we obtain the action URL from WooCommerce itself (which, ultimately, uses WordPress’s get_permalink() function). If it “knew” that the cart page was served via HTTPS then it would provide us with an HTTPS URL … of course what we see here is that it doesn’t do so.
Probably then the best way forward is to augment this tweak by filtering on post_link (and forcing the URL to HTTPS if it relates to the cart page).
As it sounds like you only wanted to keep us in the loop I’ll go ahead and close this thread – but thanks again for sharing, it could definitely be useful information for others in the same situation 🙂