Anybody can SDC. The $50 fee is waived for GM+.
Nothing wrong with asking about inventory on the flight, and nothing prevents an agent from telling you if its almost full or open. That same info is available from expertflyer.
Its not required to call and let them know you'll be missing the first flight, but I prefer to and have it documented in the PNR so in case somebody looks to cancel from a no-show, its there that it is an SDC.