..and clearly shows that the one leaving the room is Benji, not the real Solomon.
Since Benji in the mask is identical to Solomon this is not the case.
We don't see Benji leave the room as such... we him stand up (which he would have to do to swap with Solomon), the next time it's just someone who looks like Solomon that we are supposed to think is Benji.
Also, Walker is watching the room from the laptop.
Certainly that's what we're meant to think but we don't actually see Walker viewing the transformation of Benji into Solomon.
It's possible, since the IMF is clearly on to Walker at this stage, that they froze the screen on what he was looking at so he wouldn't see the swap or, more likely, it's just that he wasn't looking at the laptop during the transformation/swap.
Mission Impossible is all about twists and turns so this is just another one we have to take on faith.
In other words, Walker (and the audience) were meant to be faked out...they don't have to explain everything.