And when you actually use your light, do you unclip the bolt-snap and hold the light in your hand, such that, if you were to happen to drop it, it could just float away?
I do not have it clipped off while using it (some really rare laziness-induced situations in caves aside). I unclip it completely from me to use it. It is much less encumbered in that manner. You shouldn't be worried about dropping the light, though. It's simply not really a problem.
Do you use enough line in attaching the bolt-snap to the light that you can use the light while it's still clipped to you? And how do you secure it from dangling when you're not using it?
No, the line is as tight as possible. The only thing separating the light from the boltsnap is the knot in the line. While not using it, innertube holds it in place and keeps it from dangling.
Related question: I read an article about how to attach a bolt-snap to a piece of gear. I think it said to use "cave cord" or "cave line" or something like that. Is that the same thing as regular paracord 550 nylon line (I.e. 550# tensile strength, typically black)?
No, cave line is DRASTICALLY different from paracord. Paracord has multiple twisted strands inside of an outer nylon braided sheath. Cave line is a simple braided line. Cave line is MUCH smaller diameter than paracord, and is typically white.
https://www.divegearexpress.com/library/boltsnap.shtml