small s is S.I. unit of time (seconds)
big S is S.I. unit of conductance (1/ohms)
If only there was some unambiguous legacy unit for Siemens which would avoid confusion with seconds, samples, and whatever else might use S which itself is easily confused with 5. Maybe it could even be mnemonic.
Centralsemi describe the 2N4392 as a analog switching or chopper JFET, so dont bother giving a mutual conductance for it as they don't expect it to be used in its linear region.
Exactly, annoyingly as it is, JFET specifications are generally only given in relation to the intended application. Some of the other specifications can be calculated but some would need to be measured.