if a qualified parachutist from another service went through selection and was badged, would he be allowed to wear sabre wings
No need to be from another service - lads from Para regiment/para units put their regt wings up straight away. Surprisingly there was the occasional RM passing selection for Hereford, and I think I once saw an RAF Regt lad as well.
The wings don't signify any greater para skills - the HALO course is just another course, and not undertaken by all at Hfd, and very few if any from 21/23.