Everyday I'm grapplin',
Grapple, grapplin' grapplin'...
- Kanati "Hunting Eagle" Uwohali
Some spoilers for "Summer at the Lake" here, so if you'd rather wait to read the transcript, it'll be up later today.
Having been stunned, grappled in the glabrezu's pincers, and dragged over in front of a prismatic wall, Kanati the barbarian is then shoved toward the deadly curtain of swirling colors - only the stun has worn off and he successfully resists! Sekeren, the glabrezu, is mightily dismayed while the rest of the party cheers!
When it's Kanati's turn to act, he successfully grapples Sekeren and then tries to suplex the demon into the prismatic wall.
How we worked this out in our game is in the spoiler block below and we're happy with the results... but how would you rule? I think this is an interesting situation and I wonder how many different rulings would be made. If you don't think the rules support Kanati throwing Sekeren into the prismatic wall given how the rules work, what would be the best way for Kanati to do it given the circumstances?
Some relevant (and maybe not relevant) details: At the time of the action taken by Kanati, Sekeren and Kanati were both grappling each other. Kanati has the Extra Attack class feature, is raging, and has a +3 bonus from Strength and a +4 proficiency bonus to Athletics which is at advantage due to raging. Sekeren is large and has a +5 bonus from Strength, no training in Athletics, and is Large. Kanati is adjacent to the prismatic wall. Sekeren is 5 feet away from it (squares are 5 feet each in the map above).
So what do you think? How would you rule? For funsies, don't look at how I ruled until you've come up with your own ruling - let's see how they are similar or how they differ! If you need additional details about the situation, just ask.
How I Ruled
My reasoning was based on the idea that both had speed 0 and both were grappled so moving a grappled creature didn't seem likely. As well, a shove didn't seem appropriate because of the positioning - Sekeren would be pushed back toward Marshal Haldon and Bagdolin. Next, there is some issue with passing through each other's spaces here. Finally, I considered the plight of poor Bagdolin (ChrisCarlson) who had just shown up in the scene after hauling ass all session long while battling vrocks just to get to where he stands in the image above. If he didn't get at least a few licks in before the glabrezu is kicked into the prismatic wall, I imagine it would have been disappointing.
So I settled on the cinematic result: A mad struggle and repositioning in front of the coruscating death-curtain which set up someone else to do the pushing.
I would allow Kanati to grapple/push Sekeren into the prismatic wall. Kanati's movement speed is 0, but he doesn't have to change position at all, he's just throwing the Glabrezu into an adjacent wall. Because the two are struggling in such a (relatively) narrow area, I might give Sekeren advantage on his check as he can brace himself against the side walls in a last ditch attempt to delay his fate.
This is a very typical action trope - the bad guy is trying to force the hero into the giant crushing machine of doom when at the last moment the hero manages to throw the bad guy in instead.
Heh, it only follows the trope because someone had some cluch rolls. It could have been very easily Kanati as a pile of goo next session. 
I think however, that I would have ruled the same as you did last night. He is unable to move effectively and as pointed out they are in a tight space.
Now, if both were at around the same low, "next hit kills" HP, I may have ruled that the weakend Demon gets tossed. A much more exciting end.
As to the OP...tough call, because the Glabrezu is filling the hallway entrance. How can you turn something around that fills two squares, when it takes 3 squares for you to be side-by-side? Or does RAW state that a size smaller can occupy a Larger's space? I can't recall offhand.
EDIT: PHB #191
Moving Around Other Creatures
You can move through a nonhostile creature’s space.
In contrast, you can move through a hostile creature's
space only if the creature is at least two sizes larger or
smaller than you. Remember that another creature’s
space is difficult terrain for you.
Whether a creature is a friend or an enemy, you can’t
willingly end your move in its space.
If Kanati had said he grappled, and then drops his legs out from underneath him, falling to the floor on his back, his feet kicking out to catch the Glabrezu's abdomen in an attempt to summersault him up over his head and into the Prismatic Wall, then now you're dealing with the vertical space of the hallway, not just the width, and would have allowed the action due to the "Rule of Cool", even though Badger would have not been able to attack.
Synjin wrote:
iserith wrote:
Bah. WWE? Pfft.
I was WWF before WWF was even cool, bro!
I'd have Kanati make a strength(athletics) contest to break the Glabrezu's grapple. Then he can use 20' of movement to push the demon back 10' and his remaining movement to circle around. He might need to push 15' if the DM doesn't let him cut the corner. With fast movement, his speed should be 40'. Then he can use his Bonus Action and eagle totem dash to push the Glabrezu back toward the wall. With 40' of movement, he should be able to push the demon 20'. If escaping the grapple only used one of Kanati's two attacks (that's how you're playing it, IIRC), then he could still attempt a shove at the end to throw the demon into the wall. The narration can be whatever makes for a good story, but it all depends on breaking the demon's grapple.
Before peeking at your solution, I was going to suggest an opposed Strength check for the suplex manoeuvre to succeed, by analogy with grapple or shove, and counting as Kanati's second attack (the grapple being his first). Just as an improvised ruling.
But your solution is better from the story point of view so I like yours better 
At first. I decided not to over-think the mechanics of squares and movement. But then I thought about it anyway. 
Because there is no facing, as such, in 5e, rotating to change facing costs no movement and you can do it even if you have a speed of zero. Even if you are grappled, you can still wriggle around and change facing. That is of no consequence normally. But in the unusual situation where you are both grappling and being grappled and you and your opponent are locked together, if you spin around, your opponent has to revolve around you. This is not easy, especially if your opponent is bigger and heavier than you, so a Strength check seems right. Rotating in a vertical plane, somersault fashion, to throw your opponent over your head, isn't necessarily harder than rotating about a vertical axis and dragging him around sideways. If you think it ought to be, then give disadvantage on the Strength check.
I'm not too worried about the squares. Creatures don't actually fill the squares they occupy.
Transcript please 
BoldItalic wrote:
People know more about my character than I do.