Well, I have found solutions for some of my woes; though one was a prescribed instruction (face palm).
1.The Library Server issues I was having (speed, m4a playback etc …) were simply due to the fact that PC2 – the client – did not have the PC1 drives mapped. This is mentioned in the Wiki, and it even says it’s important, but since Library Server kind of works right out of the box for most files, I didn’t pay due attention. There is also a box that is checked by default in the Media Server Options about playing a file locally if it can be found identically on the local setup (access to the file must be through an identical path on both PC’s, which means that PC1’s drive “K” must be map to a “K” also on PC2 etc …). The existence of that checkbox should be mentioned in the Wiki article on Library Server, as it was on the very second that I saw it that I fully understood the importance of mapping. Now, Library Server runs like a gazelle!
2.Mono/Stereo is not upmixing to 7 channels on my system using JRSS mixer through WASAPI on HDMI. To be fair, a couple of movies do, but that is all. Nevertheless, this is what I did to solve the problem:
Upmixing works when using channel cloning instead of JRSS through my WASAPI setup. But the problem is that using cloning means that the surround channels are much too loud. Enter Zone switching (which I already use for Audio vs Video). I sorted my TV and Movie files by channel numbers selected them all and entered “Mono or Stereo” in the Notes field. My Notes field is empty in all my files but this can also be done with a custom tag. I then created a zone called Mono/Stereo where cloning is selected in the DSP settings and where room correction is set with much sifter surround channels. In Zone Switch, I added a rule (I already had a “Video” rule using JRSS which upmixes 6 channels to 8 channels through a "Video" zone), which stipulates that any file with “Mono or Stereo” written in the “Notes” tag must use the Mono/Stereo zone. And placed that rule above my Audio and Video rules. Voilŕ!
Using “Notes” is very innocuous and allows one to choose precisely which files need to be treated that way, which is far better than creating rules about channel numbers. It took no time at all to implement. Perhaps JRSS will work without the rule in a later implementation of WASAPI in JRiver, but for now, this fits my needs perfectly: the two types of video files each have a zone with a different mixer and different room corrections.
I am very indebted to JRiver for having created such a flexible piece of software. Exploring all of its features is well worth it.