1) What receivers are compatible with the Vortex?

The Vortex 250 was shipped during a time when SBus was still a common receiver protocol, and SRXL-2 was the latest and greatest from Spektrum.
ImmersionRC's Ghost radio system can be bound in SBus mode, and is 100% compatible with the Vortex 250.
Recent open source alternatives, such as ELRS, are CRSF-only, and cannot be used with the Vortex family.