I certainly agree with you on congest, TRex. While it's true that the word is used mostly in the form congested, congest (and congesting) are used fairly frequently too, and wouldn't sound odd or unusual to most people. For example, in Forbes magazine: "Construction workers are bused to the site so that their cars and pickups don't congest the streets."
Gnostic seems a bit more borderline. As mkenuk says, some senses of the word normally use a capital letter, so a person who has encountered it only in such contexts might expect it to be completely unusable in the puzzle, let alone a common word. However, in its lower-case usage, meaning relating to knowledge, especially esoteric mystical knowledge, it pops up in all sorts of contexts: cinema ("Their gnostic faith belongs to no known dogma" - Time magazine on the films of Ingmar Bergman); political satire ("We should wipe the gnostic smirk of self-righteousness off the faces of the moral buttinskis." - P J O'Rourke); and baseball ("Here was the distilled gnostic wisdom of the mound." - James Traub in the NY Times).
So, on balance, I think gnostic ought to be made common.
However, I'm unable to agree that scion, cosset and cosseting are particularly rare. Scion is used fairly often in journalism for a younger member of a distinguished family. An example from the NY Times, from just a couple of days ago: "While Mr. Cuomo’s job approval ratings as attorney general are strong, he still appears to be best known as the scion of a political dynasty." (But the spelling cion is definitely rare - in the US and anywhere else.) And cosset, cosseted and cosseting are all used fairly frequently. For example, cosseting was used about 11 times in the last 12 months in the Guardian and 7 times in the NY Times. Cosseted is used much more often.
Mkenuk's comments about tocsin and cession seem to have some validity. Both are the sorts of words that you would find in most dictionaries, but they do have a rare feel about them. What do people think about these two?