Hi @oberstet,
Thanks for the feedback on the your experience with OpenZeppelin Contracts Ethereum Package. I am sorry that all the information you needed wasn’t in the one place.
We have some brief documentation but not as explicit as they could be to cover your three points.
https://docs.openzeppelin.com/upgrades/2.8/writing-upgradeable#use-upgradeable-packages
I created an issue: https://github.com/OpenZeppelin/openzeppelin-contracts-ethereum-package/issues/99
I added this feedback to: The Future of contracts-ethereum-package