I'm looking forward to it! And to anyone wondering what this is, it's part of ODK Insiders!
For the first scenario, I don't think you'd want to use constraints because those would prevent you from making changes. You won't be able to apply a style to the field itself but you can have a styled message that only appears if the value has changed. You can also do things like share the old value and ask whether it's still accurate. Here are a few ideas: https://docs.google.com/spreadsheets/d/17rqStJer5SaDUazxTgpp3FnJpBDOKtnndwhgwE9mpek/edit#gid=1068911091
I used an in-form list to make the example easy to try but everything should work exactly the same with a select_one_from_file and an entity list named trees.
For scenario 2, it sounds like you want to capture a new value and make the field required. If you need to save those values to the Entity, you would use the save_to column. Does that sound right?