That said, I (and pretty much everyone I have played with) have had enough common sense to know when to throw out the RAW in exchange for a RAI as makes..well..."common sense." Ergo, far as I can see, Stun=no action. NOT "Stun=No action except in the event you have to dodge a fireball."
Well in this case it is clearly not RAI but maybe rules as I intend (the FAQ quoted earlier)
Also take into account the "RAW" are not, nor have ever been meant to be, set in stone...especially when they contradict the "realism of the game world" (the "verisimilitude" I see used here all of the time) or "common sense."
True but not relevent to this issue since you are now classifying almost all conditions as being the same, i.e., not allowing any actions which is clearly not the RAW nor RAI.
OTOH, someone who is Prone is thought of to be helpless, yes? I would see no reason that they should lose any type of save (well, as long as they're conscious, I suppose).
Nope prone is not "helpless". Can someone delvier a coup de grace on someone who is prone? No way in any common sense world.
Heck you can be prone for "defensive purposes" to help against ranged attacks.
Note that saving throws are not "actions" as defined in D&D. IF you consider them actions what type are they?
Can you make more than 1 saving throw a round?
If they are actions then you are making more actions than allowed per the RAW.