There are so many ways to argue this.
am just curious, from the practical view point, what is exactly wrong with Left & Right boots are not part of the same pair, if they were produced to the same spec (apart form the obvious tongue issue that you highlighted)? Looking at your pictures they look exactly the same from outside. And I guess most human being will have their left foot different from their right foot anyway and they would not be able to tell.
(I know that thing will be annoying when we have noticed it, but clearly you did not notice it before)
EG can argue that the boots were discounted before they were labelled incorrectly + the tongue.
If they fit, I would keep them but think it would be pretty difficult to move on a pair of what is in the end, odd boots