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After a day of trying, I'm *almost* at the stage where I understand the interactions between FreeMiNT system configuration, EasyMiNT symlink hell, and SpareMiNT init system.
API Mismatch 
Ah! One of the Lightning people replied to my post on the atari-home forum tonight: The MiNT developers changed USB driver internals, and they are incompatible with the current Lightning driver distribution. Which, in turn, is missing one essential file (no one noticed up to now).
Well, I still have the rest of this day to finally get this going...
@galaxis
What kind of USB adapter do you use? I'm guessing this is for your TT?
...still not quite there.
MiNT kernel dropping into shell after the SpareMiNT init can't load my current XaAES...