It would be better to add “one” rather than “it.” That’s because “it” implies a specific or particular key, while “one” implies any object of the type “key.”

No you can not add "it". That would be grammatically incorrect. You would have to rewrite the sentence so that you first told us what "the it" was. "it" and "that" in an English sentence usually refers back to something "A thing NOUN" earlier in the sentence. "Interestingly, of all the wallets that had a gold coin inside more wallets containing no gold coin were returned than ones with it" <-- but this is a not very helpful sentence for any real learning purposes. "Interestingly, of all the wallets that had a gold coin inside more wallets containing no gold coin were returned than ones with the gold coin" <---- correct "Interestingly, of all the wallets that had a gold coin inside more wallets were returned without the gold coin than ones with" <-- it is much better for your sentence not to use "it"

I’m addicted to watching Netflix. Not me. I can live without. It seems cleaner and clearer to finish this way. In your example, it doesn’t hurt to add “one” or “a key” but it isn’t necessary. “It” would be wrong.

"Interestingly, more wallets containing a key were returned than the ones without (it)." It is optional here!