How does the Heat Metal spell interact with a suit of armor, which could be taken off in parts?

Question

Heat Metal may be cast on "a suit of heavy or medium metal armor".

Plate Armor is a suit of heavy metal armor. It "consists of shaped, interlocking metal plates to cover the entire body. A suit of plate includes gauntlets, heavy leather boots, a visored helmet, and thick layers of padding underneath the armor." We know from their being referenced elsewhere that gauntlets and helmets are objects in their own right.

A druid, on her turn, casts Heat Metal on a Knight's armor. The Knight takes 2d8 fire damage. Regardless of the results of his Constitution Save, he cannot 'drop' his armor on the druid's turn.

On the Knight's turn, he removes his helmet (free object interaction) and takes off his gauntlets (use an object action) and casts them aside. In just 99 more rounds he can finish doffing his armor.

On the Knight's Squire's turn, his Squire, who is in leather armor, picks up the Knight's helmet (free object interaction) and puts it on (use an object).

On the druid's next (subsequent) turn, she can "use a bonus action...to cause this damage again."
If the druid does so, what happens, and why?

1. She cannot cause damage to anything, because the "suit of armor" as it existed is no longer an "object" (and perhaps it is now three different objects).

2. She can cause damage to the Knight (through the remaining armor) and the Squire (through the helmet).

3. She can damage the Knight or the Squire, but not both.

4. Something else / none of the above.

Answer 1

The rules don't cover this, so the DM must make a ruling.

Doffing heavy armor takes five minutes, and that is the highest resolution ever given to the act of doffing heavy armor. The rules do not cover removing individual pieces of complex armor, so when the DM rules that the Knight can remove his gauntlets and helmet within the action economy of a single turn, that is a ruling the DM has made because the rules don't cover the Knight's chosen course of action. It falls under this clause of the Actions in Combat section of the combat rules:

When you describe an action not detailed elsewhere in the rules, the DM tells you whether that action is possible and what kind of roll you need to make, if any, to determine success or failure.

Since this course of action is not covered anywhere in the rules, just as the DM had to rule on the attendant action economy of the course of action, the DM must make a ruling in the further implications of allowing the Knight to take this course of action.

I know this isn't a satisfying answer, in the sense that I provide no guidance to making the ruling, but that's how it goes sometimes. The three rulings you provided all seem entirely reasonable to me, simply because the rules don't speak to this situation.

And finally, this sort of conundrum is at least anticipated by the authors, as we see in the introduction to Dungeon Master's Guide:

The rules don’t account for every possible situation that might arise during a typical D&D session.

Answer 2

Full doffing does not take place for 5 minutes. Unless a house rule covers the individual parts, this situation cannot happen.

While this may seem like a realistic scenario, full plate is one object and the doffing does not complete unless there's a house rule about individual parts. See the description of the spell:

Choose a manufactured metal object, such as a metal weapon or a suit of heavy or medium metal armor, that you can see within range. You cause the object to glow red-hot. Any creature in physical contact with the object takes 2d8 fire damage when you cast the spell. Until the spell ends, you can use a bonus action on each of your subsequent turns to cause this damage again.

The object is not removed so no one else should be able to don it.

However, if anyone is touching the armor for any reason, the damage does happen

The are plenty of normal situations where creatures would be in physical contact with metal heated outside of the target. Let's just say instead the knight is riding a mount in full plate which is contacting the mount. The mount is considered a creature and is in contact with the object being heated. The mount being a creature would also take damage and is subject to the effects. If the situation was instead the squire was helping them take the armor off, then they would take the damage since they are making contact(though this is also subject to the next section).

DM house ruling will make this situation a lot more complex and require complex rulings

If a DM does allow taking off armor off piece by piece like this, then most likely they will need to rule the object you are targeting which will be up the to DM themselves. Going down this road can be extremely complex and have side effects. For example, imagine Barb the barbarian is grappling this knight, being able to target armor not in contact with Barb meaning she will not take damage. This has a lot of balance implications when how a spell works is modified even slightly.