x264 is a DCT-based codec, so everything always gets converted into the frequency domain on a per-block basis, which of course can introduce rounding errors in theory, especially at 8 bit. It’s possible they have ways to avoid this, but it’s something to keep in mind.
Yes. This is why I’m not convinced that x264 is truly lossless.
Yeah, me neither. But it’s certainly good enough for almost any scenario imaginable, except maybe some scientific uses where it really needs to be bit-perfect.