Kretschmer identified both Illyrian and Messapic as satem languages due to the change from IE o to a.
That sounds like complete nonsense to me. How does the argument go?
How does he know that PIE 'o' always changed into 'a' in Illyrian? I have studied Croatian toponyms quite a bit, and a generally accepted etymology is that Colapis, the ancient name for the Kupa river, meant "winding water", that "col" (winding) comes from *kwol (an ablaut of *kwel, meaning to turn), so, if that's true, not all 'o'-s changed to 'a' in Illyrian.
And even if we take for granted 'o' changed to 'a' in Illyrian, how does it follow that Illyrian is a satem language?