The key word for me here is "willingly."
They're not "willingly" being pushed back into another person's square. They're being shoved back by a Cube in an attack.
I'd say that shove means if they pass, the row 1 people effectively shove the person behind them back as well. Possibly causing them both to fall, as person in row 1 column A collides with person in row 2 column A.
But you can't look at the action at the level of the Row. You must look with more granularity.
I would look at it like this:
- Person in Row 1 Column A (r1cA) passes her save and is forced back five feet. This means the cube can move forward into that square. More on where she goes in a moment.
- Person in Row 2 column B (r2cB) fails his save. He is engulfed and the cube can move forward into that square.
So that's the first piece. The cube has advanced 5 feet, engulfed 1 person and driven 1 person back.
- person r1cA is now in r2cA's square. Oops. r2cB now needs to save or trip. r1cA probably needs to do the same. If they fall, can they still make saves against the engulf? Or just at negatives?
- person in r2cB is threatened and makes his initial save. Pass or fail, doesn't really matter to the Cube; it advances.
At this point, the Cube advances into row 2's range. r2cB can either save of fail; doesn't really matter to this discussion. What matters is how r1cA and r1cB are treated.
As a GM, my play here would be a save against falling for both of them. If they fall, then they're prone and at much greater risk of engulfing. If they pass and still standing, then they are shoved back out of the way.
Digging around, I can't find a definitive Monster Manual based answer here. But, there is a little piece from one of the published adventures for D&D 5E.
From Out of the Abyss (Spoilers, obviously)
Taking place in The Oozing Temple encounter, which is steadily flooding with water,
we get this little snippet:
Development: If Glabbagool is with the party, the intelligent gelatinous cube floats upward as the water rises and squeezes through a crack in the ceiling to escape the flooded temple and remain with the characters.
The only other possible answer I can give you is rooted in Science, which has a shaky relationship with a Fantasy world anyway.
In order to float, you must have a lower density than the stuff you are trying to float in. Average density is mass divided by volume. Of all things in D&D, a Gelatinous Cube is literally the easiest possible thing to compute the density of.
We know that an average gelatinous cube is 10 feet on a side...1,000 cubic feet. And, according to the 3.5E SRD, a Gelatinous Cube weighs 15,000 lbs. Divide that out, and that gives us a density of 15 pounds per cubic foot.
Convert to metric for comparison purposes and we get 240.277 kilograms per cubic meter.
Fresh Water has a density of 1,000 kilograms per cubic meter. (Salt Water is even more dense)
Thus, a Gelatinous Cube has a lower density than water. According to RL Science, it should float, and float pretty high in the water, too.
Best Answer
This old issue of Dragon 124 might help a bit with your question. It was a resource back in the day, though it's not 5e. You asked for any resources including those using old editions--and that article is a treasure trove of useful info you can use.
One particular bit on page 57 of the issue:
I would posit, based on this that the cavities or bubbles are maintained within the creature, and that upon death, they deflate and the creature loses cohesion.
It's DM call on if the acid of the cube still has an effect after it's "killed." What I have done, as it loses hit points is allow it to puddle in the hall around the PCs. For about a round it remains acidic--but that's a DM call there. As they take away the hit points from the creature, it does naturally move toward warmth vibration and sound, so even if it is a bit shorter, it will try to block their way.
Once it's dead, all that mass has to go somewhere, and I work it like a gel-filled balloon that's been popped. But--you can work it the way you did, ruling that it maintains cohesion for up to an hour after death or more, or less, that's your call as the DM.
I have also had some fun with PCs once they have defeated the creature by having the gel start to coalesce again--remaining bits into baby cubes. The PCs were so freaked out by it shifting to come together again that they simply ran away, despite the fact that the baby cubes would have moved away from them as they are now too big to eat. That isn't at all canon--I just like throwing something their way they haven't seen.
Not all cubes are going to be the same--I use the MM as a jumping off point and assume that some creatures in an isolated dungeon might have developed slightly differently. It also keeps my players from metagaming too much--if they are of a particular level, they expect specific creatures in their range, which is annoying, because they will try to use out of game knowledge to fight said creatures.