Aha. Nope. At this point, I'm guessing that this Interlink device (an old one) is different enough from the current one(s) that Windows 10 doesn't "see" it properly, in that the device shows up, but the part of W10 that can fetches stick and switch positions, etc. isn't working - and that RealFlight 8 uses some other API to get that information (and thus works properly).
Thanks for your attention to this, even if the result is negative. My actual RC transmitters (both FrSky devices running OpenTx, a QX7 and X9D+) have USB connections, so I'll try those and see how it goes.