There aren't any specific rules; this is not such a rigorously developed thing as to have them. I do think you might be overthinking it a bit.
In 2014 we had some super cool hats but then they vanished and I lost a couple of screenshots I took. In 2015 I just opened a thread so we could preserve some memories (as screenshots) and laid down some suggestions to keep it orderly, and to make it fun to participate in and not a competition or an exercise of public judgement. I left an answer and to my delight others joined in as well. I've done the same every year since.
Fundamentally I started it to be about the hats you've worn and seen that you liked. I dunno what you'd add that isn't a screenshot, there is no must/mustn't include, there are no rules about the scenarios you've asked and I'd prefer there not be any; policing things would compromise the nature of this as a fun exercise. If you're excited enough about any hats (locked or unlocked) you'd like to do some editing you have my blessing; nobody's going to win anything and there's nothing at stake so if you want to see your avatar in a hat you don't have yet and show how great it might look there's no harm done.