I think you'd make a great owner, I read your introduction post shortly after you wrote it and again just now, and I love your philosophy on animals, your cage idea, and the story of the beloved hamster you had as a child!
I think that your research is correct in that DE would not kill any mammal except if ingested in a huge, huge amount probably, and you're right that the intestines are lined so it's not 100% likely to do internal damage if it's simply nibbled on.
However, you do have to consider the extreme sensitivity of a guinea pig's stomach and digestive system. They are physically unable to vomit, so if they get an upset stomach, they will just simply stop eating which means they stop eliminating, which means their whole digestive system stops working, sometimes within as little time as 6 hours, though 12 hours, I think, without eating or pooping is when it becomes a life or death situation.
Before that point, you must start hand-feeding them (either pellet mush or Critical Care, a liquid rescue food) with a syringe, which they usually fight you on, until their digestive systems start running again.
Often times, even with hand feeding, they get too weak and/or lose too much weight and will still die. This sounds like an extreme unlikelihood, but it's not. So many things can cause them to get gas or an upset stomach and they just quit eating. They lose weight alarmingly fast too, so you don't have much time to save them.
They are one of the only animals I know of that will literally starve themselves to death because of upset tummy.
Again, I'm sorry if I sound harsh or anything, I don't know if you knew about their propensity to starve. I have some experience with this, unfortunately.
In closing, I'll repeat that I do think you'd make a good owner, you've already doing better than most people (myself included) who don't research enough about guinea pigs before taking them in. I researched some, but not for weeks, as you are doing. I am still a brand new owner compared to most people here and don't know near enough about these dear little creatures.