The connive ability still resolves even if the creature leaves the battlefield
The Fire Crystal makes your red spells cheaper, which helps you to double-spell and keep turns explosive, and it also lets your creatures attack the moment they hit the board, so you stay on the front foot
For example, consider Twinstrike from Dissension
You can focus on Ad Nauseam if you want, but you can also build a less single-minded deck by going demon-focused
In this Modern age of Magic set design, staying true to the flavor and story is very important to Wizards , which means if a card doesn't happen to be on theme for a set, it doesn't make it into the set, even if the card is potentially important for making Standard function as intended
This is an incredibly loaded question that can have several answers