One thing I actually wanted to comment on a while ago, but forgot is with regards to the Guardian Shield: I think the Pocket Mirror should be added earlier in the process, specifically as a part of the Ankh Shield. The reason for this primarily has to do with how radically the addition of the Paladin's Shield changes the usability of the item. See, the Ankh Shield is something that everyone is going to want to make, if they can. Stacking as many immunities as possible on one item has no downsides whatsoever. The Paladin's Shield however basically makes you a tank in the MMO sense, which is great if you want to be a tank, but somewhat detrimental if you don't want to do that.
Now, before I continue, I realise that this entire issue applies only to co-op multiplayer. For a singleplayer character this is all entirely irrelevant, no need to tell me that.
If you play as mage, ranger or summoner class, your armour wont have the highest possible ratings, and you'll favour damage over defence to begin with, so the thought of using an item that makes you absorb damage from your teammates can seem very unattractive, even when you figure in the 6 additional Defense. That means that you're effectively standing there with your Ankh Shield and having to decide; do I upgrade to Guardian Shield and take additional damage whenever I'm in a party, or do I keep my Ankh Shield and forgo immunity to Stoned.
If the Pocket Mirror was made an Ankh Shield component rather than added in the last step, this issue would not exist. The Ankh Shield alone would provide all of the relevant immunities, and the choice to upgrade to Guardian Shield would solely involve whether you are willing to play a tank. It would also be more thematically appropriate given that all the other debuff immunities are part of the Ankh Shield. Lastly, it would give you the ability to add Stoned immunity to your composite accessory relatively early in Hardmode, when Medusas are still very dangerous, rather than having to wait until after Plantera when they are no longer all that threatening.