1) Ah hah hah hah, good one. We just make stuff up. Katherine took a physics course once and instantly forgot all of it. Michael's a high school drop out. It's why we write fiction: it requires so few actual facts. By odd coincidence we were just sitting here quizzing Jake (our son, not the character) on a physics book he's reading. He's home-schooled for now, so we wanted to be sure he's actually reading the stuff. He just rattled off a five minute spiel on probabilities and light hitting glass and of course we understood not a word. But we were convinced he read the book.
2) Michael was always the sci fi guy. Katherine was the animal person. (Oversimplifying a bit.) M read all the old classics -- Heinlein, Asimov, Clarke, Dick. But actually a greater influence was probably philosophy, especially epistemology. We both studied a bit of existential phenomenology and always were concerned with the nature of experience. That probably had the greatest practical impact since we were very interested in the experience of seeing the world through animal eyes.
3) To tell you the horrible truth we don't remember. By the end of REMNANTS we were utterly burned out and running on fumes. We'd written something like 5 million words in ten years, we had a baby after many years of being a childless couple, we were getting no sleep and we sort of staggered to the finish line like delirious marathoners. We realize that's not a pretty picture, but you need to understand that in addition to Animorphs and Remnants we had Everworld and all the earlier series we had written -- endless numbers of books, characters, plotlines -- all crammed into our heads. And because Jake was a lousy sleeper and had health issues we were getting 4 hours of sleep a night. That last year was a sort of blur. There's a reason we quit when we did.
The passed along question: We never speculate about what would have happened in some theoretical continuation of a story. It's the reader's job to decide what happens next.