Can the Soulmonger's Tentacles grab & drop a creature into lava the same turn?

Question

In the Tomb of Annihilation adventure, one of the last encounters is against the Soulmonger, a cylindric evil artifact suspended over lava by three adamantine struts.

The artifact can defend itself through its tentacles:

Tentacles. Once per turn, in response to any creature making a successful melee weapon attack against the cylinder or one of its struts, the Soulmonger attacks that creature with one of its 30-foot-long tentacles. The tentacle bas a Strength score of 22 and makes one melee weapon attack against the creature: +7 to hit, 24 (4d8 + 6) bludgeoning damage. Instead of dealing damage, a tentacle can grapple its target (escape DC 16). If the target weighs 330 pounds or less, the tentacle can also lift the grappled creature and move it to any unoccupied space within its 30-foot reach, or drop it in the lava.

Can a tentacle grab and drop a creature into the lava during the same turn? Or is it intended two take two turns (one to grab the creature, one to drop it)?

Normally, a grappling creature could grab a character, drag them somewhere, and drop them all in one turn, but I'm unsure how the tentacle are supposed to behave.

Note that the PCs could be level 11 at this point, so a drop into the lava is unlikely to kill a melee character outright.

Answer 1

All the described effects happen as part of the attack

The text is a bit tricky to parse, but I think the word "also" ultimately means that every described effect happens as a part of resolving a single attack. This sentence makes it clear that the tentacle can choose either damage or grappling when it hits with an attack:

Instead of dealing damage, a tentacle can grapple its target (escape DC 16).

And thanks to the word "also", the sentence after that describes additional effects that the tentacle can cause when grappling at target (emphasis added):

If the target weighs 330 pounds or less, the tentacle can also lift the grappled creature and move it to any unoccupied space within its 30-foot reach, or drop it in the lava.

In fact, weirdly, a strict reading of RAW implies that the only way for a tentacle to move a creature is to hit it with an attack, even if it already has that creature grappled from a previous attack (unless there is additional text not quoted here that describes other actions the tentacles can take).

Answer 2

It can drop them on the same turn

There is nothing in the description that indicates that it would take two turns.

As you state, even a normal grapple could achieve this. This also is a climactic battle at the end of the campaign for tier three characters, so the opponent can be expected to have some devastating attacks.

Answer 3

No

The feature states (emphasis mine)

If the target weighs 330 pounds or less, the tentacle can also lift the grappled creature and move it to any unoccupied space within its 30-foot reach, OR drop it in the lava.

So to me it seems clear that once it grapples an opponent it can lift and move them, OR it can drop an already grappled target into the lava.

I would argue that it can probably drop a target normally on any turn, but I suspect this is done deliberately so that players have a chance to free a grappled character before they get dropping into the lava. Specifically because of the 330lb limit the only characters they can probably move are the squishies, who would likely die in that lava.

So getting grappled becomes a rush to escape, rather than a death sentence.