RS-422 is a differential electrical interface standard, while MIDI specifies both a current loop electrical interface and a high level protocol, the latter of which is probably its most defining characteristic today as MIDI can run over any serial interface. Today most commonly it runs over USB or its own current loop signaling, and has less commonly in the past been used over RS-232 or RS-422 from time to time. This is to say, it’s a stretch that MIDI and RS-422 are cousins. (Standard MIDI interfaces typically seen with the 5 pin DIN connector are not RS-422 if that is what you’re thinking)
It was probably RS-422, because RS-422 as a transport for MIDI back in the day was not uncommon as a desktop serial interface (like on old Macs and their lineage), but neither was FireWire later and now of course USB, that’s why I wouldn’t refer to them as cousins any more than USB and MIDI are. RS-485 is electrically similar (and in some aspects compatible) to RS-422 but with provisions for multi-point networks due to switchable drivers. The MIDI spec and the signaling is not related to either.
Sure whatever, to be clear the RS-422 port on a Mac was a generic serial port not a MIDI port per se. If you were running a PC at the time you would often use the 9 or 25 pin RS-232 port for the same purpose (which is the same port used for a modem or some mice). A number of period devices would have a selector switch between RS-422 and RS-232 for a single 8 pin mini DIN input.
Of which there are none because RS-422 is not a protocol. And if you’re talking about MIDI the electrical interface part of the spec (ie what can actually be compared to RS-422), not the protocol part, it is a point to point isolated current loop system - it is hard to be much different than RS-422 a differential voltage multipoint system.
That you can run the MIDI protocol over RS-422 is no more notable than that you can run it over smoke signals or semaphores.
In practice almost every common short to medium range interconnect has been used commercially for MIDI at some point, including SCSI, Ethernet, bespoke fiber in addition to what was already said.
RS-422 is a differential electrical interface standard, while MIDI specifies both a current loop electrical interface and a high level protocol, the latter of which is probably its most defining characteristic today as MIDI can run over any serial interface. Today most commonly it runs over USB or its own current loop signaling, and has less commonly in the past been used over RS-232 or RS-422 from time to time. This is to say, it’s a stretch that MIDI and RS-422 are cousins. (Standard MIDI interfaces typically seen with the 5 pin DIN connector are not RS-422 if that is what you’re thinking)