Beatmaps can only use 4 to 9 keys. Anything else is not properly supported.What does "not properly supported" mean? Other key modes play properly in the game client.
Long notes that are held simultaneously must be released at the same time. Long notes with multiple release timings require coordination that players of this level cannot reasonably handle.Forbidding this pattern for Easy and Normal seems a bit excessive to me. If there is a progression in difficulty from Easy -> Normal -> Hard, players should attain the muscle memory required for basic LN patterns before they encounter some more advanced LN patterns in Hard maps.
I think this should be a guideline for Easy instead, and restricted only to LNs that are released with the same hand. For normal, make it a guideline that restricts different release times if they are in fingers that are next to each other. Also, for both difficulties, there should be a minimum of time between one release and the next one (about 2 seconds for Easy and 1 second for Normal sounds fine to me).