I don't know about [1] and can only tell you regarding [3] that the basic gain of the preamp is quite high--more than I've ever needed. But it's also nearly as quiet as it is physically possible for a preamp to be.
As for [2] there's no problem running the preamp at full gain, but I really don't expect that you'll need or want to do so with amplified music and condenser microphones.
The thing is, for most mike preamp circuits, if you're the manufacturer and you want to get the best-looking noise specification, you turn the gain all the way up and make your measurement that way. But few people actually use a preamp that way unless they're recording very quiet sounds at considerable distances and/or are using microphones with low output (e.g. ribbons). I think that the spec is written the way it is because the engineers at Sound Devices were trying to give an idea of the preamp's performance under real-world conditions; they're like that.
--best regards
P.S. #1: A few years ago I spent a day performing noise tests on every preamp I owned. I decided to test each preamp at the same gain level--a moderate setting that I typically use in live recording (ca. 35 dB). I found that the preamp with the worst published noise spec (the FMR "Real Nice Preamp") was actually one of the quietest at that gain level.
P.S. #2: Preamp noise can easily become a fetish. Your microphones (even dynamic microphones that contain no active circuitry) have a noise floor of their own, and of course every venue does, too. What matters is the amount and kind of noise that you could possibly hear when playing back the finished recording. Professional condenser microphones typically have output noise voltages in roughly the -110 dBu range, and it shouldn't be hard to find a preamp with a noise floor 6 to 10 dB below that at all audible frequencies. Worrying about preamp noise when it is completely covered by microphone noise (which in turn is completely covered by venue noise) is an unprofitable use of one's time, attention and equipment budget.
--best regards