Is there a reason why is "йди" not accepted? AFAIK, in Ukrainian "і"/"й" at the word beginning are interchangeable to aid pronunciation, am I wrong?
There's no reason. You're right about і/й.
Can this be interpreted the same way we say in colloquial English to "not go there" when it comes to bringing up an unwanted subject.
Woman: "I want to talk about what happened yesterday."
Man: "Don't go there."
[Background terror music]