じゃない in this case functions as a confirmational rhetorical device, like, 'isn't that right?'. This can occur after verbs, nouns, and adjectives (in positive and negative forms).
ことない isn't written as ことはない simply because it is more casual speech, and は frequently gets dropped in casual speech （越したことはない = 越したことない).
Does ないで get inserted between こと and は?
I assume you mean 「～こしたことはないではない」？
じゃない is the casual form of ではない, and the first は has simply been omitted. Again, trademarks of informal speech.
ない／（ん）じゃない rhetorical phrasings:
寒｛さむ｝い＋んじゃない？Isn't it cold?
寒｛さむ｝く＋ない？ Aren't you cold?
寒｛さむ｝くない＋んじゃない。 It isn't cold, is it.
田中｛たなか｝さん＋じゃない？ Isn't it/that Mr. Tanaka?
あの人｛ひと｝はもう来｛こ｝ない＋んじゃない？ That guy won't come again, will he?
来｛き｝てくれる＋んじゃない？Aren't you coming?
You can add ですか? after じゃない to strengthen the positive presumption aspect somewhat.
田中｛たなか｝さんじゃないですか I believe that is Mr. Tanaka.
Conversely, having a question mark at the end increases the unsure and speculative nature of the rhetorical ('isn't it.' vs. 'isn't it?').
Does it show the speaker is upset? Or is it a form of feminine speech?
It is not a form of feminine speech. It is not an indication that the speaker is upset, the exclamation point would be a likely indicator though.
Related post can be found here.