This is kinda a shameless selfpromotion but also involves some of the questions here.
t/186787I made a topic based on some answers I found here and myself about some skin element behaviours during gamelplay and more.
It has notes about HD-elements, beatmap skins, some basic animation notes, a list about skinnable elements and skin.ini lines, a section about the mania key files and more. It is stickied and since it doesn't get any responses I don't know if this is all there is or I left out obvious stuff.
My goal was to list some of the answers to have a quick overview about eventual problems and there solves.
It mostly has standard gameplay covered but other gamemodes are in need of facts.
Some points may overlap with the OP from here but mine aren't really as detailed as here.
I hope by posting here to get my topic a bit more alive since it is kinda related to this topic.
Thanks for reading this and eventually my topic.