1. Three different things
SSR is a coded, actionable request the airline must answer (WCHR, VGML, BSCT) and it gets a status like HK or KK. OSI is information only, no reply expected. Remarks stay inside the agency and never reach the airline.
AMADEUS
SRWCHR/P2Wheelchair request for passenger 2SABRE
3WCHR-2.1Wheelchair request for passenger 2TRAVELPORT
SI.P2/S1/SSRWCHRWheelchair request for passenger 2