possessives – Double Genitive: How to Use ‘A Friend of Hers’ Correctly?

Yes, it's fine. You can use multiple pronouns as long as they don't create ambiguity. In this case, there should be no ambiguity as yours must refer to the listener, while hers must refer to a third (female) party.

Even a sentence like the following is grammatical:

They both have good cars, but his is better than his.

To make this sentence unambiguous, though, you would have to point or gesture toward the people you're referring to as you say the pronouns aloud.

Really, it's just a matter of making sure you're communicating clearly.

one should love everyone's wife

Well ... it is grammatically correct. However it does not mean what you think it should mean. What this says is that you (or someone) should love everyone else's wives. I'm sure Cassanova, Don Juan, or Donald Trump would agree, but most other people would not.

If you want to say that every person should love his own wife, then you could say:

Everyone should love his wife.

The challenge with this is that it is not gender-neutral, and nowadays in many places it's possible for same-sex couples to be married. Unfortunately this leads to some awkward grammar, something like:

Everyone should love their wife

Not as elegant as we might like, but we do the best we can.

(Edit) A better gender-neutral version would be:

Everyone should love their spouse.

This takes into account marriages where neither partner considers the other the "wife".