Not all Lucky Blocks require Rebirths to unlock, but several of the higher-Luck ones do, which is why an account without enough Rebirths can't purchase every Block from the shop yet.

A Mutation like Cyber or Void is valuable because it boosts a Brainrot's income generation well above a standard version of the same Brainrot, making it worth more both to use and to trade.

Yes, you can still get Secret-tier Brainrots if you miss a Limited Time Event, either through low-odds standard rolls or by trading for one in the Trading Hub, though both routes take real time.

Yes, you keep the Lucky Blocks and Brainrots included with my purchase, exactly as shown in the listing.