Mill also has the advantage of letting you play small mill creatures with upside to gum up the board; There's a lot of small creatures with deathtouch or grab a card and mill so you're not readily open to be attacked. Any that can also be a different card type like Nyx Weaver or Millikin will make them slightly more relevant with Muldrotha out as well.

The baffling nature of Mark's comment aside (reducing the product load by cutting the 4-5 commander decks a year but putting 2-4 in every set, on top of 6 sets a year now???), I'm honestly torn. Some of my favorite commanders are from the era of set-themed commander decks, and for every forgettable deck we get a Shorikai, Genesis Engine or Mirko, Obsessive Theorist, which are not just strong but give support to certain mechanics or strategies the colors didn't have a good leader for. If we accept that product fatigue is a lost cause, I'm not necessarily against themed commanders; even bad sets can have a shining piece of salvage like Mirko. (I'm betting it's on the fact that he doesn't have a hat.)

In this case, the ability to cast them is tied to the cards themselves, not Neyali. From her rulings:

• The effect allowing you to cast the exiled card if you attacked with a token continues to apply even if Neyali leaves the battlefield or another player gains control of it. It also continues to apply during later turns—as long as you attacked with a token during those later turns.

Compare Hedonist's Trove to Gonti, Lord of Luxury. The enchantment exiles cards and then has a second linked, but separate, ability to cast them. Blinking the Trove would "empty" it and you would lose access to the cards. Gonti, like Nyali, exiles the cards and then gives the cards the innate ability to be cast. So long as you meet the requirement of attacking with a token, and are within timing restrictions, you can cast the card.