The difference between a Hammer and a Gemstone is that Hammers are cosmetic weapon skins used only by the Beast, while Gemstones are worn by every player and serve a real function, letting the Beast drag knocked-out Survivors or warning Survivors when the Beast is nearby.

Not through normal Crates, since retired event Sets are removed from the drop pool once their event ends, which is exactly why they hold extra value and are usually only available through a shop or trading.

The difference between a Crate and a Bundle is that a Crate gives a random Hammer or Gemstone skin, while a Bundle is a guaranteed, specific Set purchased directly with no randomness involved.

Yes, you keep the Sets, Hammers, and Gemstones included with my purchase, exactly as shown in the listing.