This is a long-standing argument in D&D 3.5e forums. We aren’t going to settle that argument here.
Your case, though, is not an issue. When shadow evocation acts like contingency, it acts like contingency, which includes the caster having the ability to cast some other spell as the contingency’s companion spell. If you wanted, that could even be another casting of (greater) shadow evocation.
The real issue, as HeyICanChan suggests in a comment, is whether or not a caster can believe their own illusion. You can choose to fail a saving throw, but does a caster even get the opportunity, in this case?
And there’s just no answer to that. RAW, the lack of any special carve-out for spellcasters’ own illusions means they are capable of falling for them, but adjudicating illusions has a lot of room for DM ruling, so in practice it’s not at all unreasonable to say that it’s just impossible. Certainly, being able to use greater shadow evocation to cast contingency when you have banned Evocation is a heavy nail in that school’s coffin, which is a strong reason to disallow the combination, but that’s not in the rules (which assume Evocation is just as good a school as the others even though it is not).
Anyway, really, contingency should just be banned anyway. The spell is almost-certainly the strongest in the game. Its existence completely changes the nature of the game, and not in a good way.