Jim,
The threat of violence coms from the possible closure of the Border (again)

If it is closed, it will not be by UK.
There is no need to close it.
Why is IR/EU threatening that and why blame UK?

The cross border trade is a tiny, trivial proportion of EU trade.
They can afford to leave it free so everyone wins.

This is all about punishment and trying to prevent a successful Brexit which EU fears above all else.
They will put every possible obstacle in the path of a successful brexit.