Regarding Cockpit Tray Tables - I've been involved with the FOEB MMEL process for over ten years and have asked Airbus this question numerous times seeking justification for the MMEL requiring the tray to be stowed or removed.
The most informative response I've ever gotten was that they believe it was stowed during flight evaluation tests and no determination has been made as to whether it was okay to have it extended for takeoff or landing. Essentially - "we don't care"
The FAA has stuck with the most restrictive MMEL option in the absence of any supporting data from Airbus. There is no other regulatory requirement in the FARs or other regulatory documents.
Don't get me started on speedbrake use to meet emergency descent parameters.