Gemara on 65b says (to quote your point-by-point email)
"Question (Abaye): May a woman scheme and use a nut to button on, in order to take it out to her son [to eat]?
(He asks according to the opinion that permits scheming [to wear multiple garments at once to save them from a fire], and also according to the opinion that forbids ...) This question is not resolved."
Why are shabbos belts assumed to be permissible, aren't you scheming to take out the key to open the door by wearing it as part of the belt?
Chaim
Chaim Shulman, Teaneck, NJ
There seems to me to be a difference between the case of the nut and that of the Shabbos belt. Even the case of the nut is forbidden only as a stringency, to quote the MISHNA BERURAH (301:76): "as it looks like she is deceitfuly carrying on Shabbos." This is because the nut is not really associated in any way with the clothing she is wearing. It is just a round object, like the coin or stone mentioned in the Gemara, which happened to be useful in this fashion. When this lady proceeds to give the nut to her child, it is clear to everyone that the nut never really had much to do with the clothing, and is forbidden as it looks like a trick just to circumvent the prohibition carrying on Shabbos.
In contrast, in a Shabbos belt the key acts as a buckle of the belt and can easily serve that function repeatedly. No one eats it after they walk into Reshus Harabim (or Yachid), as is the case by the nut. It therefore does not promote the appearance that someone is merely trying to get away with carrying on Shabbos as much as the case of the nut (which in itself is a stringency).
Kol Tuv,
Yaakov Montrose