I imagine it from the viewpoint of the observer. If I construct a world in my computer with a conscious observer within the world, the world that that conscious observer sees is actual with respect to that observer. So where's the holes in that construction?

How do I know he's conscious? Because I observe him and he says so at some point. Is there a better test of this? I'm pretty sure the model would hold up to an alternate test. But then again, does it matter if he's conscious? What makes the world any less actual to him if he's not?

So I modify time in my view of the world. I look at things in different times, going back and forward, adjusting speed. All of these things modify my view of his world. But my internal observer's view is not modified. If the internal observer's view is not modified, does that mean that it is actual to him independent of my view? Is it actual to him independent of my construction altogether?

Were we to follow that into our world, I think that modal realism would then predict the uncertainty of physics at the quantum mechanical scale, because the uncertainty and related probability density functions would come about as a consequence of every possible way to maintain our continuum of consciousness of the actual being randomly realized. As you reach the more small and less-significant-to-the-macro levels you would expect that if our world was one of many possible that there would likely be a subset of many possible worlds that would maintain a consistent representation of our world at the macro level.

Really I'm not clear that causality, perception, or anything like that necessarily has anything to do with possible worlds. It can, but I don't think the time, events, space or dimensions are necessary preconditions of a world.

As to solipsism, you might look at modal realism in the lens of solipsism as a constraint on reality to the set of possibilities that maintain such a continuum of perception to allow consciousness. I just don't have enough confidence in my own ability to remember or perceive to give solipsism primacy as the important problem of rationality. Probably a luxury of having a scatterbrain and bad memory.

So is modal realism the metaphysical bridge from math to physics that describes the nature of existence? Or what? :)

There is a strong possibility that things exist we have not even imagined on any scale of science. Metaphysical levels could very well be out of our present and future ability to analize scientifically. Origion of our universe,others perhaps, and other dimensions. Our own universe is stranger than we can imagine. How did all this matter from subatomic to galactic superclusters organize. All this knowledge is currently of limits to us.

I think that modal realism describes how our world exists without requiring a further framework. I do not think it is trivial or a subset of another question relating to perception. I think it is a sufficient description to provide for the existence of our world (not to mention any and all other possible worlds).

As to hypokeimenon I really don't think there's much to the framework or really anything that needs to be dreamed up to make it work. I don't know that conscious perception is necessary to define existence, but I think that because I have it and you purport to have it is sufficient to show that something can exist. Isn't that enough?

It seems reasonable and true to define the existence of an arbitrary A with respect to an arbitrary B to be true provided there is a relationship between A and B of any kind. That is, we can define the existence of objects in terms of the existence of a relationship between that object to other objects. Neither has to necessarily exist with respect to an arbitrary C, because we haven't defined a relationship to it, but each must exist with respect to one another because there is a relationship between them. Is that fair to say?

The cat paradox is just cool. It can't be taken literally of course because there's far too many interactions going on to get an object the size of a cat to exist in both states without interacting with the world in some way, especially if we're trying to keep the cat alive! But the basic idea of our world being fundamentally fuzzy at the smallest levels is very well established. Did the post of the guy who brought that up get deleted? I don't see it anymore. Hmm... my lack of memory of the details of that missing post (and question as to whether it was ever there in the first place) is actually a pretty good metaphor for some of this stuff! Both of us are interacting with it and so have a relationship with it, so I suppose it must have existed. Now if only I remembered what it said...

I can't help you on why to bother wrestling with questions outside of our own knowledge and perception - that's outside the scope of things I have opinions about - except to say, why not?

What is the benefit of increased attention on the simple elegance of nature? What is the need of such a concept of simple elegance of nature?