By gross Yes or No
As described on page 216 of the BG-DiA, an infernal war machine is "made" and "made of iron". It is therefore a fabricated metal object, therefore a valid target.
However, it also states that "the statistics of hellish war machines work like that of a creature", which means that you could argue that they count as creatures. So there is some DM discretion on whether to treat him like a creature. So ask your DM if he treats him as a creature, it is a valid target for creature targeting spells. If it is not, it can be targeted by the heat metal.
This could be argued both ways. He says, "like those of a creature", which would involve treating like a creature. But at the same time, functioning as a creature would imply that it is not a creature and that it should be treated more like a wielded magic object.
Personally, I would lean to treat them like creatures, there is an aspect of balance, many spells cannot target objects, which limits how you can fight one.
If you treat it like a creature, it cannot be a target for heated metal.
Now suppose your DM indicates that he treats it as an object, you can answer some of your other questions.
Note that nowhere in the spell is it said that the object suffers damage, so the vehicle itself will not be damaged if it is a question of ;an object.
Can they continue to pilot it:
Well, you can continue to hold a sword on which it was thrown if you save successfully, so you should be able to continue flying if you save, you will only take damage each turn. I would also say that, just like armor, you continue to suffer damage if you stay inside, internal passengers would flee or be damaged. External passengers can simply let go like a shield weapon. But does that get into the gray area of what happens when you throw it on an iron ladder that someone climbs or on an iron cage with a prisoner? The fate leaves a lot to the DM.