a mechanism for tainting the stolen / confiscated coins
That fucks up fungability.
Surely it does not fuck up fungability of any crypto assest simply because such a crypto asset has not been yet devised. No protocol to my knowledge envisages this.
This mechanism would simply display a message that a legit owner of a crypto asset has tainted / locked / whatever his coins. Does it fuck up fungability? - I don't think so. Thieves might still exchange the tainted coins between themselves. Nobody could force you not to accept payment in tainted coins.
a mechanism for making the stolen / confiscated coins unspendable
That means Messing with the coins in someone else's wallet, rendering the coins useless after someone else has received them.
That does not mean messing with the coins which belong to someone else (that means ''messing'' / doing whatever pleases you with your own coins)!
It is perfectly fine to have a crypto coin's protocol that envisages for all the miners not to process transactions with tainted / locked coins.
a mechanism for making the stolen / confiscated coins retrievable by the victim
That means to be able to reverse the transaction with which the coins were stolen / confiscated.
That does not mean to be able to reverse a transaction. It only means the person who had originally tainted the coins (while still in his address / wallet) can in any point of time un-taint the coins which were / are in use by the thief.
Imagine the following situations:
1. You are a burglar. You are breaking into my house. When you do it I taint my coins. You put a gun in my head, I give you my private key. You and your buddies leave my premisses with my two cars. The next day you call me and tell ''hey, I stolen the coins that I cannot do much with unless you un-taint them''. They I say ''hey, I've got a proposition for you: bring one of my two cars you stole to me and I will un-taint a certain amount of coins for your kindness''.
2. You are doing business as an ''ethical authority'' in Iran. You caught me doing something illegal, e.g. selling alcohol. You confiscated my margarita drinks and my crypto coins. You want me dead for selling margaritas. I place you an offer. Let me live and I will un-taint my coins for you.
3. You are a kidnapper. You want my money. You and your mates are shooting at me, at my children and at my bodyguards. Imagine a real mess. People are bleeding to death. I happen to carry a small wireless device. I press a button and taint my coins. I can now negotiate. If you release my kids, I can un-taint some of the coins, if you release my wife (who is needed in a safe location to countersign the un-tainting of a larger portion of coins), I will un-taint another portion of the coins, etc.
These are just hypothetical examples showing that tainting the coins can be useful for people.
Please research if it is legal to destroy state currency in your jurisdiction (you will find out that it is not) and then ask your government how this is a crime, not me. I can only tell you that it is.
1. I have just painted my 5 euro bill black. I hear no police siren sound. Nothing happens. I wonder why
2. ''Your government'' this and ''your government'' that - you know what? I don't have any government.