So, is there anything in the book that I missed, that suggests the different physics are related to the soft ones' nature?
Ja.
Från del II, kapitel 3b, kraftigt förkortad:
Odeen said, "I can't explain it all, you haven't had the background. I will try to make it simple and you just listen. You understand, first, that everything is made up of tiny particles called atoms and that these are made up of still tinier subatomic particles."
"Yes, yes," said Dua. "That's why we can melt."
"Exactly. Because actually we are mostly empty space. All the particles are far apart and your particles and mine and Tritt's can all melt together because each set fits into the empty spaces around the other set."
Odeen hastened onwards. "In the other Universe, the rules are different. The nuclear-force isn't as strong as in ours. That means the particles need more room, because they spread out their wave-forms more. I can't explain better than that. With a weaker nuclear-force, the particles need more room and two pieces of matter can't melt together as easily as they can in our Universe."
[...]
"If the nuclear-force gets a tiny bit weaker, then the atoms take up more room and then what happens to melting?"
"That would get a tiny bit harder but it would take many millions of lifetimes before it would get noticeably harder to melt. Even if someday melting became impossible and Soft Ones died out, that would happen long, long after we would all have died out for lack of food if we weren't using the other Universe."