You keep trying to make it that AST's main mechanic is supposedly the cards. It isn't. AST main mechanic is healing (just like the other two healers since SE doesn't count healer DPS when they create content).
An AST not using cards is like a SCH not DPSing when they have time or not using a fairy, a BRD not using songs, a NIN not using Huton,a DRG not using heavy thrust, a BLM not using enochian, a WHM not using any DPS spells when no one is taking damage. Can you complete the dungeon? Of course! It isn't using the job to the full potential, but this game isn't tuned hard enough outside of savage raiding for that to matter.
So really your gripe seems to be more about the lack of difficulty than the cards. If you want the cards to make a difference between success and failure then the content needs to be way more difficult where every bit of damage/TP/MP/defense/cooldowns counts. So you'd have to turn all content into savage raid level difficulty or maybe even harder...
"it shows they are an unsuitable mechanic for a fast moving and hectic group encounter." I'd like to know what content you did as an AST that brought you to this opinion? I play AST in current savage raids and I make plenty use of the cards. So they haven't been unsuitable for me.
Anyway, what OP is asking for is not a whole rework of AST or their cards. Only simply asking about making it so you cannot overwrite cards so you are bringing this thread off topic. You could just post your own thread or bump your other thread in which you already said all of this before.
I think there would be issues with OP's suggestion if it wasn't properly implemented. Like for example say I was in a group as AST with another AST and they AoE a card half way into the fight. Now my MP starts getting low and I pull an Ewer (thank god right), but I still have the other ASTs card on me....if it doesn't let me overwrite then I cannot even place my own card on myself. Another example would be if the other AST AoE or uses Ewer on players who do not need it and I draw a better card like Balance or Arrow...some of the DPS don't need Ewer but I can no longer overwrite any of them to give them an actual beneficial card. I'm just not sure if this can be implemented properly ><
SCH does get non-stacking barriers but you do get a bonus of having both fairy types and each of them as automatic Embrace healers. One SCH summon Eos and other summon Selene and then you get both buff pools (or you could have both summon Selene on Obey and trade off using Fey Wind for a practically constant speed boost). It pretty much makes up for the non stacking barriers. Plus at 60 SCH has way more direct healing utility to make it so double SCH isn't really penalized that bad. Emergency Tactics and Idom are really awesome.