If the connections are solid it wouldn't pick up any more noise than a 6.5mm to RCA adaptor would have. Any jiggle to the cable would result in popping or cracking though, depending on the configuration could be quite jarring.
No more than a standard connection. Every cable inherently acts as an antenna, so that’s why we try to avoid running them parallel to power lines and other things that would give off audible interference. If you actually want to reject interference, you’d need a balanced signal. Regular RCA and 1/4” are both unbalanced, so they’ll both pick up interference regardless of how they’re tied together.
When dealing with unbalanced cable, the most important part is making sure your signal to noise ratio is good. If you can get a hot enough signal that your gain can be lowered, you may be able to reduce the interference completely below your noise floor. Of course there are arguments against this (like how running things that hot could potentially mean you’re clipping your outputs, which introduces a whole host of other issues) but as a general rule, you want your gain to be as low as possible, so you can reduce the amount of background interference and noise you’re picking up.
I'm still very surprised that the most rudimentary of antenna can transmit and pick up signals acceptably. Like everything else in reality, I imagined that it'd have TONS of noise but somehow it's all still audible.
You don’t always have the luxury of time though. I work in entertainment, and I’ve definitely scrapped adapters together in a pinch. When you have a show starting in 15 minutes and a musician rolls up with some bespoke gear with weird connections, your only real choice is to bodge something together and make it work.
Nothing quite like seeing five adapters chained together, to go from stereo RCA to TRS 1/8” to TRS 1/4” to dual TS 1/4”, to XLR… All because you didn’t have a direct box that went straight from RCA to XLR, because another musician walked off with it after their show wrapped up at 2AM last night.
As long as whatever is on the other end doesn't reproduce anything at 60Hz or does so with a lot of loss and there are no heavy vibrations closeby (such as the speakers themselves), it should be fine.
It's the one part of the radio spectrum that's also in the hearing range hence induced currents from it can be heard in an analog audio system and which would have a pretty decent amount of energy inside a building because it's being emitted pretty much all around by the mains power wires.
That said, you're right that depending on the length and shape those wires might not form a very good antenna for that frequency. Independently of that, I doubt there's enough energy around to directly affect speakers, but it might be enough if that wire goes to an amplifier which in turn powers the speakes.
That said, I'm from Digital Systems rather than Telecomms or Audio so take what I say on this as a semi-informed guess.