Hi, firstly "なくて”here is conversational expression compared with ”なく”by adding "て”, secondly " は”　here is an emphasis. Thirdly " じゃない”is more conversational than"ではない”which is formal. Please note the letter "じ”in ”じゃない" has two dots over the letter "し”.I think these forms are interchangeable.

It become だけでなく when you abbreviate the word だけではなくて(so they removing は and て) but still it's the same meaning. Using だけではなくて is more suitable when it comes to formal writing although you can still use だけでなく depends on how you want to express your thoughts or how you want to explain in your writing, だけじゃなくて is more casual and can be used in friends conversation or other situations. I think we use だけじゃなくて more often than the others. However, you can also use だけじゃなく but it more appears in casual writing. １・文章を読む時一字一句だけでなく、全体の意味も考える。 (Both だけじゃなくて and だけではなくて can be used) ２・図書館は平日だけではなくて、週末も開けています。 (Both だけじゃなくて and だけでなく can be used) But if you want to use だけじゃなくて, 「図書館は平日だけじゃなくて、週末も開いてる or 開いてるよ」sounds more natural. In this case If you want to use 敬語 at the end of the sentence (like the 開けています/開いています) だけではなくてand だけでなくshould sound better. 　 ３・これは女性だけじゃなくて男性も使えるよ。 Same as the sentence above. if you want to sound more formal it become 「これは女性だけではなくて/だけでなく, 男性にも使えます。」

なく formal, written なくて casual, colloquial 1. why the second and the third ones need the final て, while the first doesn't? ↓ Because the first one is a formal expression which is used in the book/sentences about the way to read. 2. is it because of the "は" in the second and the "しゃない" in the third one? ↓ No. 3. Are these forms interchangeable? ↓ Yes and No. Even though you change those, the meanings do not change. But the nuances are different.