"And in the time Gar has been shipped with her he's been seriously weakened as a character"
As if he wasn't written as such, anyway. I mean, he is the sort whom bad stuff happens to all the time--loss of parents, military-minded adoptive family, being labeled a green-skinned freak by many regular persons, the entire Terra arc, Raven's turning evil, et al--and yet, he is still here. If they had wanted to kill Gar, they could've done so already several times (ex: Slade's fatal gunshot wound that Rae saved him from, Terra's going insane and attacking all in her path, the seeds--as mentioned--killing him when the Sons of Trigon first appeared, the fact that he is one of the few Titans with a close-range attack range (which makes him easier to hit).
"(failed leader,"
Actually, he's done a decent job as leader, despite Rae's problems. In fact, the only recent things seem to look bleak is because that is the extent of his character. He is the happy-go-lucky guy to whom bad stuff consistantly happens to.
"seen as lovesick,"
Ummm...he is in love. Being lovesick is practically a requirement.
"deaged,"
So what? Raven's been deaged constantly. If she was at her real age as a character, she would be years older than practically anyone. But because of her demon genes and constant rebirths from such, she is able to regenerate herself. Seriously, she is caught between heaven--which won't admit her--and hell--which she adamently rejects--which basically strands her on Earth for all time. And as long as those seeds in the changeling persist, he'll receive the same treatment, most likely.
"obsessed with her in a way that is identical to Wally's behaviour)"
Except that Gar is fully in control of his faculties. Unlike Wally, Rae is not manipulating Gar's mind. Out of all the love interest both have had they balance each other out, because they both have inner struggles, they both faced down the Terra situation, they both love each other in a way that doesn't seem like a false love pushed together (i.e. unlike the one-sided Jericho issue, the forced times with Wally and Dick, the clearly evil nature of Eric Forrester, and all the easily forgotten Terra-like replacements [i.e. Jillian], these two complete each other, beyond the whole "keymaster" thing).
As the whole "portal" and "key" thing with Trigon, it is not unlike
Ghostbusters [
[link] ], where Gozer uses Tully and Dana Barret as his vessels, before becoming the Stay Puft Marshmellow Man and eventually his fully realized form (who looks like a girl, to be honest). But in the end, the busters defeated him. For the Titans, the same will be said of Trigon, since the good guys always win in the end all scheme of things.