Also one wonders what is done with the 'evidence' after the case. I could imagine the FBI simply deleting the wallets but does anyone know their actual policy?
In the US, law enforcement gets to keep a portion of whatever they seize. You can bet your ass they will not destroy millions worth of assets -- entire departments will get bonuses for years for this.
Law requires them to liquidate the assets through auctions, they cannot simply sell them at an exchange, or set a price for them and sell them at that price. There are plenty of companies that specialize in organizing such events.
Pretty sure whatever their policy is, it isn't "throw away $150million of seized assets"
What does the FBI or other agencies do with intercepted drug money and criminal possessions? Drugs get destroyed, that much I know, but not sure about the other stuff.
I imagine the btc will be saved until the case is closed then try to be used for other investigations, I don't think the feds would try converting and selling it.