Hey Miroslav,
the new APIs are finally here! As I explained for your coffee machine, you can use the new bridge API to better integrate with Octoprint’s login system.
Once you’ve done that, you can use the new activity log metadata to implement point 5 and 6 of your roadmap: Attach any print details to the activity log to keep track of material usage, print duration, …
We originally planned to allow only key-value pairs, but @roland convinced me it would be useful to support full JSON as metadata – so we did. I hope you like it. 
Unfortunately, the activity log (and therefore the metadata) is only visible in the admin app for now – normal members can’t see it yet. But were planning to add the activity log (and much more) to the member portal in the foreseeable future.
Let me know if you have any questions.