Modern legal and ready to play!
While a wider global release hasn't been confirmed at the time of writing, both Scarlet and Violet and Sword and Shield's DLC ended up getting a physical release later down the line (albeit with some content as a required download), so we wouldn't be surprised to see Mega Dimension follow suit at some point
However, Lost Zone Box is likely dead as is Lugia Vstar, two top-tier decks from the past season
Medusa's last ability resolves before the spell that caused it to trigger
(To proliferate, choose any number of permanents and/or players, then give each another counter of each kind already there.) The value of X is determined only once, as Tromell's last ability resolves
Second, based on the rumored reprinting time frame of Modern Horizons 2 and Wizards mentioning off-handedly that Lord of the Rings will leave Magic Arena in 2025, it's pretty safe to assume that whatever deal Wizards worked out for Lord of the Rings allows them to do new print runs of the set for the next two years, which would make the release similar to Modern Horizons 2 , at least as far as reprint potential