You didn't ask a question! You wrote some sentences with errors in them. It's not clear if you want corrections, or if you want to know if the idea is correct. But, to correct them: If we didn't know you, we would think that you were insane. (But the tense choice is a bit odd) unless we knew you, we would think that you were insane. (In this one, the tense/conditional logic is very odd - it doesn't really make sense, and I'm not sure what you mean. If you are just making up an example, you should use a different one, or tell us the wording of the example) His brother speaks much more politely than he does. His brother speaks as politely as he does.

"Unless we knew you" would be fine in conversation (though a little awkward: be better to say "if we didn't know you."). And it isn't very common/idiomatic to say "would decide" here--we'd say "we would think" (or "we'd think"). His brother speaks more politely (or if it's a lot, "much more politely") than he does. Same level of politeness: His brother speaks as politely as he does. Never "much politely" together.