Is it possible to Twin Spell Dragon Breath or Booming Blade?

Can you use the Feral Variant from SCAG on the variants in Mordenkainen?

What’s the in-house opinion about options from Sword Coast Adventure’s Guide that weren’t put in Xanathar’s?

A firbolg PC casts Booming Blade then uses hidden step to go invisible…

Is the attack made as part of Booming Blade magical?