'Yet' is to 'already' as what is to 'eventually'? Could 'never' do?

I don't know if my title serves well in expressing what I am actually trying to ask, so maybe an example will do better:

I have done it already. <-> I have not done it yet.

I waited patiently, and he eventually returned. <-> I waited patiently, but he didn't ??? return.

My non-native-English-speaking intuition tells me that the same word eventually wouldn't be appropriate in the negative sentence, the same way as already wouldn't in the first example. Is there any truth in this? Assuming there is, I would normally use never + Past Indefinite to express the "opposite"¹ of eventually

I waited patiently, and he eventually returned. <-> I waited patiently, but he never returned.

However, I started having doubts whether this usage of never expresses what I used to assume it did, the reason being that I was unable to find this meaning of never in the dictionary (I checked Dictionary.com and Merriam-Webster). According to Dictionary.com:

never can be used instead of not with the simple past tenses of certain verbs for emphasis ( I never said that; I never realized how clever he was ), but this usage should be avoided in serious writing

So, even if never is the word I am looking for (which according to the definition above it is not), it's said to be informal.

I am hoping someone could point to the word I am looking for, if there exists one, and also throw some light on this usage of never (for example, which "certain verbs" is Dictionary.com referring to? Does my example sentence sound natural?)

^{1 What is 'yet' to 'already', by the way? Is there a special term?}

Ever is the word you're looking for. Both ever and yet, along with any, are Negative Polarity Items.
In fact, ever is suppletion for *anywhen, which is not allowed in English, although anywhere is.

It isn't never, which is a negative trigger itself, not a polarity item.

Since an NPI can only occur in the scope of a semantic negative, it's the not in have not done it that licenses yet; it would be ungrammatical if there were no negative.

• I have not done it yet (grammatical)
• *I have done it yet (ungrammatical)

Similarly,

• I waited patiently, but he didn't ever return.
• *I waited patiently, but he ever returned.

Negative polarity items are all idiomatic and often have very strange syntax. When you encounter something strange in English grammar, look around to see if there's some negative word or construction that's causing a disruption.