I was only answering the first question. Since you were talking about wanting to essentially strip off one channel, I assumed there was no sample rate change. However, I guess if you’re converting sample rate it’s because you need to, so obviously pay attention if you have quality choice available. But rate conversion is easy when not in real time, so I don’t expect it to be poor quality.
If you want more detail: The amount of noise added, in dithering, is always relative to the sample size being truncated to. In Audacity, the wiki says reduction to 24-bit adds -130 dB peak noise level. (IMO, they shouldn’t bother, since you can’t hear the difference between truncating and dithering at that level.) But at 32-bit float, it should add nothing. Even if it did, you could never detect it with your ears. Really, the only place you’re going to get in trouble is if you export as 16-bit, so I wouldn’t worry about the dither aspect. But personally I use 32-bit float audio tracks, so that’s my choice anyway.