To be fair, you are unfamiliar with the software and not in a position to distinguish a bug in the software from user error. Harmony (or even TB Studio) is not a program you can jump into and do much with before taking time to systematically learn how to use it. It helps if you take it in manageable bites and expose yourself to a variety of sources to see it from different perspectives…broad to focused, back and forth. You will spend most of your time frustrated facing a wall otherwise…at least initially until you begin to understand how it works and things connect. The manual won’t warn you of all potential problems that can occur when you do something wrong. If you do something wrong you have to search for the problem and hope someone else made a similar mistake and the solution is available online or hope someone responds to your plea for help. You are better off taking time to learn how to use the software before deciding whether there is anything to complain about.
I have Premium and am not sure about everything that Essentials does NOT have.
You want to use Motion Keyframes and not the Stop-Motion Keyframes when you desire computer interpolation.
It sounds like your drawing substitution problem could be related to the way your pivots are set up. Read through the first links and watch the videos. The discussion probably does not address your situation exactly but it should help you see how things can go wrong. A light may go off and lead to your exact solution. Do not get hung up on the version of software being discussed. There are more similarities than not between them and it is unlikely to be a waste of your time. I even watch videos in foreign languages I do not understand. There is something to gain in everything out there.