Thanks for sharing indeed. But whether it all is related ? ... to me it doesn't look so.
The "Allocated but can't play" message comes from Kernel Streaming, and it tells that a device was found, but it couldn't be set to the bit depth / samplerate implied (by the track + your settings like Double, etc.). That really is all, and how your defrag can influence that ... to me it seems it can't. But quite some other things could have been wrong at the time (to your imagination, and knowing that you now know/understand what the message means).
The ideal Split File Size depends on almost too many things to explain (but I have a topic in mind for these kind of thing);
The preferred setting is easy : the larger the better.
I'm not sure myself what the differenc between ideal and preferred is, but let's say ideal in this context is what brings the best performance from your system, whereas preferred merely is about the inherent useage of the setting itself. The setting is there to prevent your tracks eating all your memory, while the more of the track is in the memory, the better it is (for pure memory playback).
Since tracks have a sheer infinite (read : undetermined) length, while memory so far always is imited - also taking into account the upsampling which lets all grow exponentially, you, as the operator can control what amount of memory is allowed to be used by the tracks, before your system explodes otherwise. That's actually all.
... But it gets more complicated if you see that loading larger portions of the track (or the whole track) can take too much time because of slow processor speed and/or just too large portions. And this means gaps.
Add to this the ultra low latency (KS) stuff, and either the (high priority) playback will again cause the trackloading to be more slow, or the (not high enough priority) playback will be interrupted by the huge amount of processor cycles in a row the large portion takes to process.
And there are several types of processes of course.
Anything else for now ?
PS: I didn't check this for typos, afraid to read it all makes no sense unless two more pages of explanation are added (which I don't feel to right now
).