(1) ''We have not discussed [whether us getting back together means us getting married].''
(2) ''We have not discussed [whether our getting back together means us getting married].''
I’m not sure why you ask about a 'reduced' relative clause because there is no relative clause here, reduced or otherwise. They are different constructions but with the same meaning, and in both examples the bracketed sequences are subordinate clauses as complement of "discussed". In (1) the subject of the clause is the non-finite interrogative clause "whether us getting back together". In the less likely (2) "our getting back together" is again the subject, though it’s not a clause but a genitive (possessive) noun phrase as subject of the bracketed clause. Both examples mean "We have not discussed the answer to the question 'does us/our getting back together mean us gettingmarried"'?
(3) ''He once walked in on his parents fighting.''
In (3), the salient interpretation of "his parents fighting" is that it’s a noun phrase with parents as head and the gerund-participle "fighting" as post-head modifier. I wouldn’t talk of 'reduced' relative clause here (the syntax is quite different) though it does have the same meaning as "… his parents who were fighting". A much less plausible interpretation is that it’s a genitive (possessive) noun phrase, "parents’ fighting". There’s no real meaning distinction between the two.
I’ve simplified those analyses slightly for the sake of brevity.
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