Till and until are synonyms. As long as the writer is consistent it doesn't matter.
They are normally synonyms, but 'not till...' is something I wouldn't use.
mine is right
I found all your exercises interesting. Thanks so much
that was indeed a tricky question.. pls do give many such question
'Until' is formal and 'till' is informal , and inversion is mostly used in writing which is formal, so 'until' should be used