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?

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