I'll give some of the motivations I found towards the end of the current blog sequence, and possibly will elaborate in the next one if the ideas sufficiently mature.

Thanks. I look forward to that.

Histories of the universe could play a role in semantics of (1), but they are problematic in principle, because we don't know, nor will ever know with certainty, the true laws of the universe.

I don't understand what you mean here, and I think maybe you misunderstood something I said earlier. Here's what I wrote in the UDT1 post:

More generally, we can always represent your preferences as a utility function on vectors of the form <E1, E2, E3, …> where E1 is an execution history of P1, E2 is an execution history of P2, and so on.

(Note that of course this utility function has to be represented in a compressed/connotational form, otherwise it would be infinite in size.) If we consider the multiverse to be the execution of all possible programs, there is no uncertainty about the laws of the multiverse. There is uncertainty about "which universes, i.e., programs, we're in", but that's a problem we already have a handle on, I think.

So, I don't know what you're referring to by "true laws of the universe", and I can't find an interpretation of it where your quoted statement makes sense to me.