I have not read a lot of Philip K. Dick. I've mostly been exposed to his work by movie adaptations like Total Recall, Minority Report, and Blade Runner. This novel fits in well with those, thematically. There are a lot of questions about whose reality is real. I did really enjoy this novel, which makes me want to look for more.
I think one thing to remember is that even in the "real world", those subjective realities are real to the subject. Mr. Silvestri really sees the universe as geocentric - the sun and moon rise and fall. He could think about the Earth rotating, but that really isn't necessary for him to go about his days. [Honestly, how many of us think about it?] He can really imagine an angry God looking down and judging people and when bad things happen, he can assume it's because they've done something bad. Similarly, Reiss can live in her paranoia and every time something bad happens to her, it confirms someone's out to get her. It's being pushed into someone else's subjective reality that's confusing.
Regarding Dr. Laws, I think he's a vital character. First, he's a physicist and his name is Laws. "Physical Laws" are how we describe the real world. He is also the key to Hamilton confronting his own subjective view of the real world.
Second, A Black man named Laws reminds us that what we think of as law, the legal construct, is not equally applied. He's seen as lesser in Silvestri's very racist world. He is sort of politely ignored in Pritchett's - not because she's racist but because she finds conflict so unpleasant. He's caricatured as a thug with a knife in Reiss's subjective view, though that may have as much to do with his maleness as his Blackness.
I think it was also very interesting how Silky (the waitress/maybe prostitute) was portrayed. She's not very important in Silvestri's world, but she is massively changed and then deleted in Pritchett's. Pritchett despises sexuality, and since that's central to Silky (and Jack Hamilton pushes the issue), she has to be removed. But, she comes back in the fourth vignette, where we first think it's Marsha Hamilton's view. She is presented almost as a threat with her excessive sexuality (larger breasts, etc.) but that also connects with McFyffe's vision of her as an object of lust. I think that was a really well-done choice by Dick, to be able to present Silky's sexuality as a threat to the married woman while concealing that it was a man's view of her all along.