Can a shapechanging creature multiattack when some attacks are not available?

Question

When the Monster Manual's "Multiattack" action of a creature with a shape-changing ability lists attacks that are restricted to one or more forms, can the creature take the "Multiattack" action while in any form, making only the currently possible attacks?

For example, the listed Actions of a Gray Slaad (MM 277) are:

Multiattack. The slaad makes three attacks: one with its bite and two with its claws or greatsword.

Bite (Slaad Form Only). Melee Weapon Attack: ...

Claws (Slaad Form Only). Melee Weapon Attack: ...

Greatsword. Melee Weapon Attack: ...

When a gray slaad is in humanoid form, does this mean:

1. It can take the multiattack action, but since it cannot bite or claw, this only allows two greatsword attacks.

2. Since the multiattack action involves a bite attack but the slaad cannot currently make a bite attack, it cannot take the multiattack action. It is limited to the ordinary single greatsword attack.

Answer 1

Option #1 is correct

The slaad makes three attacks a round. The multiattack ability is not limited to the slaad form, only which attacks it can select as part of that ability.

The slaad can only make one bite attack as part of the multiattack. This is not possible while in humanoid form so the first of the three possible attacks is foregone.

It can also make two claw or greatsword attacks. Since the claw attack is limited to slaad form only, it only has the option to make greatsword attacks while in human form. It gets two of these.

So, while in humanoid form, the slaad's multiattack ability effectively grants just two greatsword attacks since both other attack options that it can use with multiattack are not possible.