ViceidolAll the four sentences refer to "past
probability"
It's difficult to understand exactly what you mean
by this comment. And even after deciphering it to some extent, it
doesn't seem true. Far from relating to past probability,
couldn't have stolen it, for example, is about an impossibility, at least in the most typical reading. By modulating the voice so as to separate
could from
not (but never with the contracted form
couldn't),
you might indicate past probability, but I'd say that that is a
secondary reading of that verb group -- a reading that is particularly
difficult after
He knew, which indicates a great deal of certainty, with which the probability reading clashes.
Secondly, none of the second group of examples is good because of the problems with the sequence of tenses. After He knew (a past), only modals that are past in form (could, should, would, might, and possibly must) should be used.
CJ