Its late but I only got the time now, sorry.
1. You should be able to sell both cards and destroy towers even after 0 placeable towers left. "Perhaps" it was buggy in your version, but it should be fixed in the next version
2. Fixed ✅
3. That shouldn't happen, if that did, hopefully it's fixed now because I haven't seen that behaviour ever
Edit2: It continues from the page you left it on. Seeing it on hex tower is expected but were you not able to click any other tower or you couldn't see other tower buttons? Was it something like that?
Edit3: The expected behaviour is you click a key, it holds that card for you to place it anywhere. Was something else happening than that? Fixed ✅ for the sell thing
Edit4: Do you mean as in upgrading damages with upgrades? I'll try to fix that