I actually got kind of bored with the Homu/Mami fight, because it got pretty samey after twenty seconds. They just kind of jumped and shot guns at eachother and blew up bullets for a few minutes straight (running on memory here idk the time) - nothing was really any different until the very end.
Although, yeah, the strange, tainted transformation scenes were cool. Homura's being all full of runes.
Anyway, yeah, magical girls age. Kyouko is depicted as significantly smaller when she first contracted in both the anime and mangas.
While the Homura love transformation thing -does- seem like a weak point, I think there's some degree of justification as well. I think it's not just that she was in love that tainted her gem; I think it's that the love was painful, for such a long period of time. Repeatedly seeing Madoka die and/or sacrifice herself, and then being in a world where Madoka no longer even existed, yet still was "there" in a way that effectively kept someone as devoted as Homura from being able to move on with her life. (This is shown in the movie, briefly thinking about it brings to mind the in-a-boat scene after she talks to Sayaka about who the witch might be) After she's a demon she does say that now she concentrates on that pain, or something to that effect.
Her life after contracting had pretty much been centered entirely around Madoka, whom she was never able to truly save, or even be happily together with at all after the first few timelines. And near the end of the movie she even actively corrupts her soul gem as much as she can in order to try and suicide to protect Madoka from the incubators. It's a pretty special case. (I still agree that the love transformation thing is a bit of a thing, though, yeah)
Also, about how Homura in the original anime doesn't make Madoka happy; that's partially because her goal of keeping Madoka from contracting or dying to Walpurgisnacht is far more important than keeping her happy during that time. As well... growing too attached to each individual Madoka in the timelines I imagine would be very painful, not to mention trying to hang out and make her happy would probably be difficult after going through that period of time so much; anyone would probably become detached after they'd run through the same event over, and over, and over.
S'kind of hard to encourage happiness when she's unavoidably tangled in the world of magical girls, anyway. Sayaka turning into a witch is implied to be a very common event in the timelines, and if Mami doesn't die by then she goes off the deep end after, even if it's not the same as it is in the anime showing of it (Some of the official side-story mangas show other ways it goes down). As well, Homura tries to push the "Being a magical girl is terrible" aspect to try to discourage Madoka from contracting.