Good points, ones that I've thought over as well. The conclusions I've come to regarding those 3 points:

1. A person eliminating toxins will pass them to others, who similarly reject them. For example if your son comes home sneezing because he walked through a storm of pollen, you might inhale that pollen and begin to sneeze as well, and the whole family might take some time to work the pollen out of everyone's clothes.
2. Same analogy as #1.
3. If these toxins accumulate, unless you clean them out 100% each time, it stands to reason that each time you become burdened with them, your body is dealing with a greater absolute burden each successive exposure/infection. Kind of like accumulated clutter in the garage, if your spring cleaning is partial every year, eventually your garage will be swamped and even one extra bag of bric-a-brac is too much. This is why the flu kills older people because of the long-accumulated toxin "clutter."

(There's also the counter that once you've become healthy, you're more sensitive to unhealthy stuff. That's like noticing dirt on a clean well-maintained floor instead of an extra spill in a barroom.)

As far as the definition of "toxins," they could be anything. Think of it like a city with a tailor, metalshop, carpenter, and a variety of other businesses: each business will have its own waste which may or may not be harmful by itself; but if the overall city trash pickup is behind schedule, any material built up in excess in one zone gets ugly fast. In the same way, every person uses their body differently, and so the buildup of waste and tolerance of unwelcome ingredients becomes different for each person based on their different lifestyles, which also helps explain why some diseases run in families. And then there are actual poisons too.

I don't really want to convince you about it, just explain that there are reasonable answers to those questions. For me it clicked after reading and listening to a lot of videos and blog posts by a guy named Darko Velcek at darkovelcek.wordpress.com. But it took me a while to digest it myself. I'm not well-versed enough to be its best advocate, but I'm pretty much on board with all his medical explanations (he gets into esoteric psychic phenomena and other fields which I'm less gung-ho about, but anyway...), which he doesn't call terrain theory but which lines up with everything that term describes. He also encourages others to learn from observation to prove him right or wrong.

I think a lot of the alternative medicine and wholistic medicine is polluted by misinformation in the same way, and with the same techniques, that clowns use to infiltrate and retake control of media narratives, which discredit anyone who attempts to leave the "Rockefeller pharm," especially if they are helping others. They already have counter-narratives set up to trap people and otherwise discredit them and steer people away. That's a long way to say that I agree that most of the debate is unproductive name-calling.

But eventually I got past it. I've been in the weeds of medical disinfo for years the same way that some anons get OCD about crisis actors or dig deep into other misinfo traps. Cheers.