Fibers serve other purposes as well. For example, fibers feed the intestinal flora. This not only helps keep us regular, but the flora also produce beneficial acids, peroxides and bactercides that help protect us from pathogens. The insoluble fibers are also our primary source of silica, which is essential for healthy bones, cartilage, nails, skin, teeth, hair, tendons, ligaments, blood vessel walls, etc.
There are different forms of magnesium though. The worst is magnesium oxide, which in water forms magnesium hydroxide. The hydroxide is caustic and burns the intestinal wall causing an influx of water and stimulating peristalsis due to the irritation. This is why Milk of Magnesia is used as a laxative. Acidified forms of magnesium though do not do this and much better absorbed.
Other forms of magnesium work by attracting water in to the intestines.
But neither magnesium, nor vitamin C are stool softeners, which is what the original poster was specifically looking for. A big difference between laxatives such as high dose vitamin C and stool softeners like slippery elm is that laxatives can cause diarrhea as where stool softeners do not.
As for the claim there is no known toxicity level that is not true. There is no known lethal level, but that is not the same as toxicity. Diarrhea for example can be considered a toxic effect since it can cause electrolyte imbalances. Excess vitamin C can also cause kidney stones in people with pre-existing kidney disease. Vitamin C also functions as a pro-oxidant instead of an anti-oxidant in higher doses to name a few.
Also keep in mind that the synthetic ascorbic acid and ascorbates commonly sold on the market are extremely unstable and rapidly break down in to oxalic acid, which is a tissue irritant and binds with certain nutrients like calcium preventing their utilization by the body.