Are the ultimate realities (paramattha dhammas) purely subjective? Or, if they exist, and are not simply subjective, why are they regularly said to be purely subjective, or subjective perceptual experiences? Where is it stated otherwise in abhidhamma?

Dear Zans, I just guess at least part of your problem is due to the feeling/notion that “space is real”.

The Abhidhamma and Commentary clearly distinguish the Suttha-method and Abhidhamma-method while accepting both. The Sutta-method only considers “Sasambhara (in general sense) mahābhūtas”.

This idea is at least very risky. A person in modern era can’t conclude that Sutta-method is the “Original method” or the “Only method mentioned by the Buddha”.

The Atthakata says both sutta-method and abhidhamma-method are original methods.

The four vinayas don’t accept personal opinions as authority over Atthakata or Abhidhamma.

If we try to define what we think as the “the thing” other than the characteristic, then we will find the answer. I guess, we will not find any nature other than the “Function (rasa)”, as the “thing”.

Hardness can also be defined as bearing, receiving etc. by considering its function or manifestation. All these are the same element.

Who does perform “bearing”? Hardness perform “bearing”.

Yes, if we know that the space is not real.
I would say as “The earth element is hardness which can bear other elements” for more security.

Kathavatthu Atthakata says that there are 3 spaces, namely: delimitation-space, kasina-ugghata-space and boundless-space. Out of these 3 spaces, only the “delimitation-space” is a Sankhata-paramattha and other 2 spaces are Pannatti.

We see as if mahabhutas occupying 3D-space, but 3D-space is unreal. Only the delimitation is real.

This force is nothing other than the function of the characteristic.
Space should be taken as conventional space.

We may say it in the conventional sense, but we have to be careful to not to take “space” as real, just like we don’t take “time” as real when we describe mind-moments.

When the body touches Pathavi, the triad “kaya-potthabba-kayavinnana” can be written as 'kaya-hardness-kayavinnana". Here the Kayavinnana is the “Sensation of hardness”.

What Classical Theravada defines as the hardness is not the “sensation of hardness” but the “objective hardness (bearing nature)”.