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
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?