According to the tobira textbook, ~だけでなく、~も~, means "not only ~, but also ~".
One example they give is, 日本語はひらがなだけでなく、カタカナや漢字も覚えなくてはいけません。

For the last two examples, the grammar phrase being taught changes to
~だけじゃなくて、~も~. The examples are

  * このアパートは駅から近くて便利なだけじゃなくて、家賃も安いから、借りることにした。 
  * その映画は面白いだけじゃなくて、音楽もいいよ。 

Do だけでなく and だけじゃなく both have the same meaning here? I'm assuming they are the
same or similar but I'm not sure. Is one a less formal version or the other?
Are they identical?

Also, in my second example, why is 便利 followed by なだけ instead of just だけ? Is
だけ perhaps a noun that 便利 is modifying?
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grammar
だけでなく and だけじゃなくて
* だけじゃなく is an informal/casual form of だけでなく.  

  * It's not "便利 + なだけ", but "便利な (na-adjective) + だけ".
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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
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The difference between でない and ではない is directly connected to the semantic
impression the topic particle は would give in normal circumstances.

ではない appears to be generally preferred in distinct, independent predicative
phrases, or when you are about to say "Something _'is'_ something, and so
that...":

  * このアパートは駅から近くて便利なだけじゃなくて...

"This apartment _is_ not only convenient being near a station..."

  * その映画は面白いだけじゃなくて...

"The movie _is_ not only interesting..."

  * 人間たちが定命の者ではなかったら、間違いなく神にもっとも近い種族だったでしょう...

"If human beings _are_ not who lay down fate..."

Whereas でない can be seen employed in other environments including but not
limited to:

  1. objects dependent on a verb phrase: 

日本語はひらがなだけでなく、カタカナや漢字も覚えなくてはいけません...

"For Japanese you have to learn not only hiragana..."

  2. direct attributives which modify nouns:

観光コースでない沖縄

嫡出でない子

This type is rather straightforward.

  3. When directly preceding, especially having an immediate causal connection to, the main event/verb of the sentence: 

ママでなくてよかったよ...

"Good that it wasn't mom...". In Japanese grammar context this use is closely
related to the one above since adjectival/adverbial constructions are viewed
as parallel, but such adverbial ones often lack a literal English parsing. In
this case, *ママではなくてよかった would have meant something along the lines of "It
wasn't mom, but (I was? That thing who wasn't mom was?) good" which doesn't
make any sense since は would have elevated that part of the phrase to an
independent level parallel to the main event よかった. To use は in this context,
you have to add extra conjunctions and say for example ママではなかったからよかった.

ヒールでないとダメ...

Similar to above. ヒールでなくダメ can also be correct, with the former putting more
emphasis on a general case "if" and the latter more of a specific instance
such as when a woman is stopped right before the entrance of a high-heel only
venue.

and one final example:

試験は優秀でないと合格できない...

which is almost exactly the same as the one above.

  4. isolated phrases where the integrity of a full sentence is not as emphasized; frequently in titles:

愛でなく

感覚でなく思考
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