I've now spent enough time on this. I'll report what I'm seeing here and give it a bump every now and then. The phone is weird.
Connection:
If I connect the phone while locked, it does not enter 'debug' mode, starts charging just fine, and once unlocked, still stays out of debug mode and all applications except for MC, can see and browse the phone's storage folder. Before MC can see the phone, it (MC) needs to be restarted.
If I connect while the phone is unlocked, all is apparently good from the get-go, but the phone drops into debug mode.
Depending on its locked or unlocked status when connecting to the PC, the phone is treated as two separate devices, with separate settings in MC for "HTC One" and "HTC One (1)"
Syncing:
Sometimes works, sometimes doesn't. Too flakey to be reliable and I'm done watching syncs and checking results. All too often, MC will try to resync all of the audio in the sync playlist. Investigation shows that this is because MC is showing the existing audio on the phone without any [Album] or [Artist] information. Checking externally shows that the tag info is present and correct in the files, that incidentally, MC created and placed there earlier.
Despite being unchecked, MC is placing a 'Folder.jpg' in every folder it syncs to on the device. This leaves a mess when 'delete on sync' actually works as the folders get left with a single, unwanted jpg file inside them.
After sync, MC writes a playlist (pls) file into a "Playlists" folder on the device. This file is always an empty, zero byte file.
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I accept that the phone presenting itself in different ways depending on its lock status is probably out of MC's control, it's annoying nonetheless and would be great if MC could get smart about it. I only have the one phone. At one point in my testing, MC was up to "HTC One (3)", though it's only ever "HTC One" in explorer.
-marko.