1)
That's an indirect quotation, so what Harry actually said is not always completely the same as 状況証拠ではそうだ. It can be, e.g., 状況証拠ではそうだよ, 状況証拠ではそうだね, 状況を見る限りそれが正しいと思うよ.
では works as the contrastive marker. It is correct on the point of 状況証拠 (although it might be wrong from another point of view).
2)
~攻めに合う is an idiom, meaning "to be swamped with~".
3)
~を…とみなす
「~が…」とみなす
Both are possible and the meaning is the same between を and が in that example sentence, but が can be misinterpreted that 彼女 is the subject of みなした depending on the sentence structure. For instance, in the clause 彼は彼女が快活な性格だとみなしたので, 彼はみなした and 彼女がみなした are both possible (although the meaning would be usually decided by the context). を can avoid this ambiguity.