I think that card wordings should be simple and reflect how you actually play (I never considered the blue dog issue you referred to to be an actual issue). Nobody puts the card in their discard pile and the moves it onto their Exile mat, they directly put it there. So „Exile instead“ makes more sense than „ gain and then Exile“.
But it is a trivial difference as you rarely want the junk to be able to leave the Exile mat (what your „gain and Exile“ Version enables you to do). This brings us back to my initial point: with your version most folks would not even be aware that you could discard stuff from Exile because they would intuitively read the card, no matter how it is worded as „ah OK, I can directly put it on the mat“.
What matters is that the wording should be clearer because currently you can easily misread the card, as arowdok did, as „gain and Exile another copy from the Supply“.
Border Wall uses the same wording as Gatekeeper. And I haven't seen people get confused about Gatekeeper or complain about its wording.
Also, with your logic, Watchtower should say "to instead either trash that card or put it onto your deck" and Replace should say "if you would've gained an Action or Treasure, instead put it onto your deck."