Can the red cavalry unit move through the gap (exactly 2 BW wide) and out the other side? In doing so, it will first contact front-corner to front-corner with both blue units, then slide edge-to-edge alongside them. I can't find a rule that seems to prohibit this. Do the close proximity zone restrictions apply?
(https://i.imgur.com/3wzqdbt.png)
My view is that the rules don't prevent this.
Although not applicable with your diagram, be aware that if you end in edge to edge contact or corner to corner contact with an unengaged enemy UG in the movement phase, they may initiate a melee combat by turning as per page 132.
Richard