As Seen2 said, most of that was due to the influence of the Kisenian Flower possessing him and exploiting his feelings of lonliness. I still consider Fiore's introduction to be a viable love interest because we're shown that Mamoru had someone in his life who he was close to before he ever met Usagi - so close in fact that they used to sleep in the same bed and Mamoru was so distraught over his leaving that he cried over him and gave him the rose Usagi handed to him as a token of his feelings.
None of this actually changes the fact the movie goes out if its way to show Usagi is the only one for Mamoru. The movie is not some anti-Usagi x Mamoru movie you’re trying to turn it into. It’s literally the exact opposite
Also the aforementioned Fiore, En, (well I guess En was really given to him by Sato and it's unclear how much influence Ikuhara had there so that might not really count) Rei and Fish-Eye.
And? Mamoru never shows interest in ANY of these characters. A non-Usagi character being interested in Mamoru=/= Showing a better option than Usagi or Ikuhara trying to show he doesn’t like Usagi and Mamoru as a couple. I’d say you’re making reaches but that’s just an understatement.
Actually you just reminded me, another scene I almost forgot - back in early Classic during the episode when Rei starts getting interested in Mamoru and tries to get him to date her, when Usagi spots them together she makes a remark on how it's inappropriate for a college student such as Mamoru to be fraternizing with Junior High School girls, so clearly episode 29 wasn't the only refrence made in calling out Mamoru's age. (And there's still that kissing episode in S where Unazuki bluntly states that Usagi is too young to even be kissing Mamoru)
It’s like you go out of your way to ignore context.
Usagi doesn’t actually care about Rei dating a college student she cares that she’s dating Mamoru (because she doesn’t like her “friend” dating a guy she thinks is a jerk, she has feelings for Mamoru she hasn’t realized yet, or she’s simply jealous that her frenemy is getting a boyfriend before she does take your pick) her reaction, like your Kenji example, is showing that Usagi is being irrational and just looking for any excuse to interfere. Even Luna adds she doesn’t think Mamoru is as bad as Usagi thinks he is. Seriously, both your examples the character with a problem against Mamoru (Kenji/Usagi) is framed as irrational and immediately counteracted by a character being treated as rational and level headed having no issue with him (Ikuko/Luna)
And Unazaki was saying Usagi was too young to be kissing in
general she even immediately adds Usagi has her high school entrance exams coming up. Unazaki does not care or think Mamoru is too old for Usagi. She just thinks as a junior high student Usagi is too young to be kissing any boy.