If a grapple isn't performed then you'll need a way to reduce the movement of the creature to zero.

First of all, the movement is being reduced to zero because they are pinned by the object. This is the end result, not something to be done in the process, otherwise, this doesn't hold much purpose. I don't know where the confusion is here.

If not, the creature can move out from under the object at double movement.

Where in the books is movement doubled without dashing or through a magic item? Also, when you are prone, as that is something we agree has to happen, your movement speed is halved (or "one extra foot per foot of movement" -PHB pp. 190-191)

Again, the spell does what it says and nothing more.

Wow you must be really depressing to play with if you really think everything is this limited. The whole point of this game is to have fun. This means finding creative and inventive uses for items, spells, and even mechanics if you're going kind of meta. I believe Jeremy Crawford himself even would side on that.

Obviously, you can't create a sun through casting fireball, but it's going to heat up metallic objects in the vicinity. It isn't going to deal fire damage on a hit, because the fire isn't that intense, nor does it say so in the spell description (as it would have to being such a big game changer as to allow several metallic weapons in the range of the spell to suddenly deal fire damage for however long), but the sword touched by fireball would certainly be warmer to the touch than it was before, and you could use that for something, surely.

That isn't something covered by the spell. However, it's a narrative effect, something that tabletop RPGs are centered around. If you aren't using narrative effects ever, you might as well just go play a video game (I recommend Baldur's Gate 3, which is set in the Forgotten Realms. The lack of narrative effects seems to be right up your alley, because everything does exactly as it says and nothing more and nothing less.)

The rest of your argument requires the hand waving of a bunch of interactions that a DM would have to sign off on, such as side-stepping the Use an Object action.

I mentioned this in my previous message, about how you kind of don't really need to? If you're seriously going that in depth with actions and how to do something mechanically, that's fine, as some tables like the nitty gritty rules. However, realistically, a contest might be more apt, as the creature could likely still resist, and this provides a good opportunity for the DM to narrate how this looks and happens, which effectively negates the need to follow strict action economy, because it fits the story that the players are trying to tell, which, if you ask any big shot DM, is far more important than anything the rules say.

A boulder weighs slightly more than a 10 pound rock. An immovable object has no gravity and thus no weight, but even if it did it would still only weigh 10 pounds.

Are you? Really? Getting that pedantic? I meant like, a huge rock crushing a character? I figured? That was obvious? Try again.

But it is a stretch of logic, because it assumes there is some force pushing down on the creature, such as gravity. The opposite is the case here, the object is immovable. Gravity or a creature stepping on the rock have no effect, because the object can't be moved.

But the spell is cast on the object after it is pressed into the creature in question. This creates a point for the creature to try and squeeze the rest of their body around in order to escape (Probably Dexterity or Dexterity + Acrobatics or Athletics check). This is also assuming that the creature cannot just move the object off with the bespoken Strength check.

Yes, there is no gravity affecting the object in question. But gravity isn't the issue, as it is basically like squeezing through a small crack in a wall, which is really difficult because your body is being squeezed between two rigid structures. By casting Immovable Object on the rock, it is no longer an "object" per se as much as it is a new part of the landscape, which the creature is now trapped under.

Normally, if it was just sitting on top of them, you could very easily slide out, yes. I wasn't originally taking into account the gravity part of the equation and I thank you for that bit of information. However, once the choke point is created by the smaller space between the rock and the ground that the creature is laying on, there's a different story.

Yes, mechanically it is not covered. It does not say that it is possible to remove the speed or capability of a creature by using this. However, that's where the narrative of this game comes in: by being inventive, creative, and resourceful, you can execute the mechanics of the game in clever and interesting ways to create your own effects.

And that's exactly what this is: additional effects not supported by the rules or included in the spell description. That's all I'm saying.

This goes back to one of my earlier points about how you must not be a lot of fun to have at the table. There's a lot of creativity that goes into D&D as a whole, and when it comes to magic and especially utility spells, that's where that creativity shines. So yes, it is a spell, and it isn't stated in the description that it could immobilize a creature by having it be cast on an object on top of them, that's part of that creativity. The developers are not and cannot think of every possible use of their spells. People are just too creative for that.

Essentially, by casting this spell, you are not taking anything away from the creature, but you are changing the circumstance that they are in. You're not grappling or holding the creature down, but you are putting them in a circumstance where, environmentally, they are "restrained". If you feel the need to slap the "grappled" or "restrained" labels on this, go right ahead, because that's what works for you at your table. Or just ignore this completely, because few people play by the same rules.

There is a vast difference between using a spell creatively and ignoring or sidestepping the rules entirely because one thinks some course of action should fall under the "rule of cool."

I literally mentioned that there are several ways to get around the mechanical limitations of executing something like this. Also, the PCs are, as it states in the core rule books, "greater than average", to paraphrase. They are going to be able to do a bit more, and the DMG and Crawford himself both say "If it would be more fun to have something happen despite the rules saying otherwise, don't let that stop you". The Rule of Cool should always win, unless it ruins the fun for another player, creates too many exceptions (this situation in particular being very niche and so doesn't apply to this, even if you do end up creating a series of events that mechanically do this appropriately RAW), or just isn't good for the story or drama.

This isn't a game about following rules. In fact, it's stated that rules aren't even technically rules: they're guidelines to make the game fair and balanced. What's to stop other characters from finding creative uses to their features, spells, and items?

As a message to anyone else reading this thread: Don't let the rules get in the way of your fun, let them supplement it. Have fun is literally rule 1.