why isn't it harmless? bc again, Kennedy conspiracy thinking leads directly into conspiracy thinking about the New World Order, 9/11, Trump, QAnon, COVID, and whatever else. it is the sine qua non of the conspiratorial style in American discourse, which is so toxic and pervasive it is, or at least is a major factor in, tearing apart our society.
yes, Oswald shot Kennedy. I don't know specifically was in his heart when he did it - no one does - beyond some kind of discontent and possibly a desire, suggested in some of the documents declassified in 2017, to impress Cuban authorities so he could successfully reapply for a Cuban visa (the idea that Cuba or the USSR would harbor a known assassin of the U.S. president is delusional, but Oswald was no deep political thinker)
I don't think that because I think people never conspire to do bad things. I think it bc 1) people, most prominently Vincent Bugliosi, have spent decades thoroughly debunking every conspiracy theory and theorist
to address some of your specific questions
1) Oswald was actually an excellent marksman - a good shot for a Marine, according to his ratings while in the Marines, and an excellent compared to the average contemporary adult male. multiple different tests - FBI, military, a doctor hired by the Kennedys, etc - demonstrated that the rifle he used was quite accurate and that the feat of marksmanship achieved was attainable both in terms of accuracy and time. Incidentally, part of the House Select Committee on Assassination's conclusion that there was "probably a conspiracy" was acoustic research seeming to show the existence of a second shooter on the grassy knoll, which was later discredited
2) he was originally arrested for shooting the police officer. the cops who arrested him were looking for the officer's killer, not the assassin. he was first charged with shooting the officer, and then after several further hours of interrogation, charged with the assassination..
3) they only mentioned his original charge bc that was all he'd been charged with. there was immediate popular speculation - i.e. beginning with a crowd outside the Texas Theater, and later the press - that he was the assassin as well. he denied both charges, which is hardly suspicious or unusual of itself.
4) I agree Ruby's motives aren't really "satisfactory", but a great deal of effort has, with little success, gone into trying to tie him to the mob in a meaningful enough way to suggest that he was acting on its behalf, so his unsatisfactory given motives and other possible factors (desire for fame, amphetamine use, etc) is what we're left with.