This is a dual audio request, which is something you obviously don't value, hence the USBD.  It illustrates why people never ask for or want any advice from people like you: people know their own circumstances and what they value, hence their preferences and what they request.  

I'm glad admin is admin.  Don't make the mistake of thinking admin doesn't know a lot more about these things than you do.

1. From the way you phrased that, I take it that this is based off some prior knowledge rather than actually comparing the specific sources here?

2. Thanks for your opinion on CRF - why is 15 specifically the threshold, but not 16? The numbers don't seem that far apart, and if 15 is just some arbitrary figure, one might assume someone could have a different preference?
If you don't mind getting into more detail, it would be interesting to hear your opinion on the other settings you alluded to.

3. Did you find any corruption in this release? If so, would this mean that all encodes using x265 3.2.1 (version from mediainfo) in the past, potentially also contain such corruption?
Looking through the changelog, I can't see any claims of changes to MVQ or similar, since version 3.2. Is there something I've missed?

4. I see. My personal gripe here is actually the usage of PCM. Lossless is fine if that's one's preference, but I see little reason to use PCM.