leather cuirass

[Dan Howard]

How do we know it was actually a subarmalis and not an outfit that was not specifically purposed to be worn under armor?

Well, that's what a subarmalis IS, I'd say! It's a garment meant to be worn under some kind of armor.

And hence my above question How do we know the above garments were specifically purposed to be worn under armor?

The alternative is that we're seeing something worn to *represent* armor, sort of a soft costume variation but not meant for battle.

Not necessarily. There are representations of a garment which is not armor but which is, nevertheless, worn in battle. A tunic, for instance. So, the above images could represent a garment which was not armor or subarmalis, but which was warn in battle.

Unless I've missed the point of the OP these items are irrelevant since they do not represent a type of armour. There are plenty of other threads about Roman clothing and what they may have worn under their armour.