@John95 I certainly agree there are quite a few expendable cameos and guest appearances that apports nothing to the story in some of Matt Selman's episodes and that they put it in the show just because they can. Even though it's not something exclusive of his episodes (Al Jean does it often too) and although I don't consider it a problem, you know, more of a detail that could be fixed. A little cameo of a guest star won't ruin an episode. At worst, I will overlook it if I don't who the invited is. I hated the hallucination scene in Bart's in Jail! not because of Bill Cipher, but because it came from nowhere, took away the realism of the episode, and gave it a wacky conclusion to the story.
We must have different conceptions of what is a happy and sitcomy ending (I haven't watched Full House so I don't get the analogy). But, I don't know, it is an unexpected ending with a last-second plot twist and a change of message (for better or worse – I don't think the lesson is "Trust no one because everyone's a scammer", but more of "Be careful because not everyone is like you", after all, Marge is a truthful person so of course there are more like her). The happy and sitcomy ending was that Marge got his money back from the girl who asked for it. The unhappy ending is the one you proposed, with Marge losing 20 dollars and her faith in humanity. This is more of a misleading ending because it looks like a happy ending because Marge got her money back and keeps her hopes in people but it is not because it was Grampa who sent the money. You could like it or not, but I don't think it's happy or cliché.
You have a point with Kirk in There Will Be Buds, he is not sympathetic there. But let's not forget who Kirk is. In few words, he is a loser with a bitter life and without friends. Neither Kirk nor Homer act with bad intentions (that's why I said there are no villains). Kirk only tries to be friendly but he is embarrassing and pathetic, that is why Homer (and the audience) feel sorry for him. Homer only tries to tolerate him while the kids play the tournament. The situation reaches a limit and Homer admits he doesn't tolerate him through a song I adore, and then, yes, it is the guy who lied about his feelings the one who has to be sorry, and not the one who doesn't know how to socialize.
I think the role of Bart in The Town is just serviceable to the story, although I can see why some people could dislike his characterization and why the ending doesn't work for them. I thought the beginning just showed Bart innocently annoying Homer for a football team, you know, nothing out-of-character, but I can see people getting problems with his influences on the family to move to Boston at first and to return to Springfield at last.
I also agree with you when you speak about the role of Marge in The Star of Backstage. I am often confused with the role of some one-time characters in his episodes, like Sasha in this one or Mary in last season's Christmas special. They are mean but are portrayed as the victims. It's not a problem that affects every episode but it's something I wish they can fish in the future.