So how are the special interest groups keeping their enemies out?

Same as it is everywhere else, collusion and nepotism.

Wikipedia editors are a special clique.

Exceptional claims require exceptional evidence?

Where is the exceptional claim?

If anything, the preposterous idea that Wikipedia is somehow the only resource immune to shilling and political influence is the exceptional claim.

No; the burden of proof lies with the affirmative. To require someone to prove that they are NOT guilty of corruption is unfair (and also irrational)