D&D 5E - Heat Metal and whether constructs are both creatures and objects?

1 How big an item can you heat with it (they came upon a metal tower they wanted to destroy).

2 Are metal creatures like an Iron Cobra or a manufactured metal warforged PC (as one of my Players is playing) objects even though they are creatures?

3 If something like an Iron Cobra or the PC is red hot from the spell, does the creature also do damage on its physical attacks using the red hot metal? How much?

HEAT METAL
2nd-level transmutation
Casting Time: 1 action
Range: 60 feet
Components: V, S, M (a piece of iron and a flame)
Duration: Concentration, up to 1 minute
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.
If a creature is holding or wearing the object and takes the damage from it, the creature must succeed on a Constitution saving throw or drop the object if it can. If it doesn't drop the object, it has disadvantage on attack rolls and ability checks until the start of your next turn.
At Higher Levels. When you cast this spell using a spell slot of 3rd level or higher, the damage increases by 1d8 for each slot level above 2nd.

I had a little discussion about this when it came up then made some rulings as the DM (medium size objects or a five foot square area max, yes they can be subject to the spell, and they will inflict the full same damage on physical hot strikes) but said I would think on it a little more because these kinds of things affected both monsters and the PC in our group and robots were a big campaign theme (I am running a 5e conversion of the Pathfinder 1e Iron Gods adventure path).

Normally I think D&D has had a line of creatures are not objects so my first instinct is no but 5e seems fairly inexact on the subject and having a double edged sword of an Iron Cobra taking damage but inflicting more as well was fun and I am trying to be more of a "yes and" style of DM and went with it. But I have concerns for the robot PC and the NPC robots that I might not be seeing a lot of relevant implications yet.

A fourth question just came to mind as well. Can you heat metal the end of a polearm or spear and have it do extra damage while safely holding the non-metal haft?

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