Hey @michihenning appreciate all that you’ve written here.
I am really concerned about your cooker giving off a burning smell. That’s not supposed to happen at all. I would let customer success know about that issue, so they can go right ahead with exchanging your cooker. It shouldn’t emit any smells (unless it’s something delicious being cooked). 
The cap can be a little bit of a stubborn one to budge. Takes a bit of oomf. Not long ago @Ember gave really helpful tips here in the community that’ll help with removing the cap. Check out this thread: Removing plastic end cap
As for the app - something is obviously not right. Not sure what exactly is going on here, but customer success can also help with troubleshooting the issue and getting you a resolution.
This feedback is super helpful - I will be forwarding this to the rest of the team. Thank you so much!
-Alyssa