In summary, I can only advise that aspiring contractors find out as much about a system/schema/policies before commiting to a contract, no matter how much they offer to pay.
An interesting perspective, thanks.
From the other side, I'd be more inclined to say, be prepared to do what is asked of you and no more, no matter how much the temptation would be to say "but if you just...".
Contractors are treated with suspicion by perm staff, partly because of financial jealousy (most salaried staff can't even dream of the sort of rates contractors will command) but also because outsiders will often point out previous poor work to the boss, often with no understanding of the historical reasons behind it.
Trying to change people's attitudes, unless you're explicitly brought in with that brief, is likely to provoke exactly that sort of negative pushback, and does little but make your life harder.