I'll add an answer to your second question, to clarify Doncot's answer which feels rather partial.

じゃない is indeed a casual form of でない

When used to modify a whole sentence, だけ attaches to the rentaikei of the previous word. That's a fancy way for saying that if a sentence has だ at the end, it should be replaced with な:

• verbs: このボタンを押すだけでいいよ。 (no change)
• i-adjectives: 話し相手がほしいだけだ。 (no change)
• nouns and 'na-adjectives': 必要なだけのお金を持ってきて。

I'll add an answer to your second question, to clarify Doncot's answer which feels rather partial.

じゃない is indeed a casual form of でない

When used to modify a whole sentence, だけ attaches to the rentaikei of the previous word. That's a fancy way for saying that if a sentence has だ at the end, it should be replaced with な:

• verbs: このボタンを押すだけでいいよ。 (no change)
• i-adjectives: 話し相手がほしいだけだ。 (no change)
• nouns and 'na-adjectives': 必要なだけのお金を持ってきて。

The word also attaches to な-adjectives with the な intact: 好きなだけ