They come from the same fruit. When the nutmeg fruit splits open it reveals a dark seed wrapped in a bright red net-like membrane. That membrane, dried and flattened, is mace; the seed inside is nutmeg. Mace is milder, more floral and more peppery, and it dissolves better into pale sauces, but chemically the two overlap heavily, including their myristicin content.

Yes, in the same way nutmeg is. Both contain myristicin, which is broadly estimated to become toxic somewhere around 1 to 2 mg per kg of body weight, roughly a teaspoon or two of ground nutmeg for an adult. At that level people report nausea, dry mouth, a racing heart, disorientation and hallucinations lasting many hours. Mace poisoning is rarer in the literature than nutmeg poisoning but it is documented. Normal seasoning quantities are not a concern.

No. Mace is graded clean on histamine content and only mildly histamine liberating, so it is generally usable on a low-histamine diet. It does carry a small amount of free glutamate and moderate salicylate, which is the more relevant pairing if you are sensitive to spice blends generally.

Only if the problem is flavour intensity. If you avoid nutmeg because of its myristicin or its salicylate content, mace does not solve that, since it grows on the same seed. A genuinely different aromatic like bay leaf or coriander seed is the safer swap when you need the warm background note without the shared chemistry.