と思えば / 合わなくちゃって / で / か

1. 「自分がどこにいるのかもわからない様子で、パーシー・ウィズリーがその場にいるかのように話しかけてみたかと思えば、また急に変わって、ダンブルドア先生に合わなくちゃって言うんです」

a) Does "Xと思えば, Y" mean something like "no sooner did he do X than he started doing Y"?

b) Is 合わなくちゃって = 合わなくては + implied いけない or similar + って (quoting)?

2. 「罠だ！君と魔法省とで、わたしをここに誘き寄せるために、偽の口実を仕組んだな、ダンブルドア！」

What is the function of で here?

3. 「おまえさんか、こうしてやる！」カルカロフはダンブルドアの足下にペッと唾を吐いた。

I suppose か is a kind of derogatory suffix here, is it?

1) a) b)
That's right.

2)
Indicating the agent. (Although 大辞泉 mentions organization/group, で can be used also for a person.)

で
３ 動作・作用を行う主体となる組織・団体を表す。「政府側―検討中だ」「気象庁―光化学スモッグ警報を発令した」

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で［格助接助］の意味 - 国語辞書 - goo辞書

3)
Yes.

Thanks.

2. I feel rather unclear here about the difference between "agent" and "subject". Is the following sentence also correct (I don't see why it shouldn't be), and, if so, what is the difference?

「罠だ！君と魔法省は／がわたしをここに誘き寄せるために、偽の口実を仕組んだな、ダンブルドア！」

Similarly with the dictionary example of 気象庁で光化学スモッグ警報を発令した, what is the difference if we change で to は or が?

Also, is this で the same で as is found in 「さあ、みんなで食べ放題の焼き肉屋さんに行こうよ」?

で can be used also in a passive sentence.
e.g.
彼と魔法省とで、偽の口実が仕組まれた。
In the sentence above, the subject is 口実. That's why I used "agent". (As you can see, で can be replaced with によって in a passive sentence.)

The meaning is basically the same as が (except such like the passive ones, of course), but が could sometimes sound "strong" comparing to で. The grammatical difference is that this で is only used for an organization/group, as in the dictionary, or a side of two or more different groups/persons. で is originally from "particle に + particle て", and the nuance is close to において "at". Probably that's why this に is used only for these specific cases.
e.g.
その案件は私の方で報告しておきます。（＝ その案件は私が報告しておきます。）
(In fact, 私の方で sounds softer than 私が, therefore this kind of expression is often preferred in business scene.)

As for the difference between は and が, 気象庁は is the topic, as you know. This sentence is about 気象庁.

みんなで is usually categorized as "state". みんなで means "all together", but みんなが can mean "you all(except me/us)" depending on the context.

５ 動作・作用の行われる状態を表す。「みんな―研究する」「笑顔―あいさつする」

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