What are the differences between colloquial English and English in common use?
If something is in common use, it could probably be used in formal contexts, while something colloquial would not.
Your understanding of "common use" is wrong.
If it's in common use, it's used commonly.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.