I am not trying to argue for the sake of saying he's overpowered, like I said in my initial post, I really am on the fence as to whether his passive really is that overwhelming. But there are a few things I felt your points needed to be addressed with.
1.) [+1] Early game, you have a point.
2.) **[X]**I disagree, actually.
With
and
he can rock many (most?) non anti-tanks, though I consider 1 AD with a +HP stat & 5 tank items to be "full tank." Sometimes I see a Garen take two damage items, but I rarely do. I always build Black Cleaver and Sunfire though. Those do enough damage to sometimes even 1v1 the ADC without much trouble (kiting and all). If you don't believe me, fine, I am really not here to say I'm right and you're wrong. I accept that my lack of trouble is just their lack of skill; maybe you'd have a very easy time against such a Garen. I am also not saying that if I am right that it's too much or needs to be changed. I just wholly disagree from my own experience that his damage is diminished enough to affect his relevance.
3.) [+1] Of course you shouldn't focus the tank.
4.) [0] I both agree and disagree.
Firstly, "If you can kite him." is loud enough statement. With MS quints, Fleet of Foot mastery, Alacrity boot enchant, and his Q, not very many people can kite Garen very far. I agree in the sense that his late game CAN BE WEAK, that depends on two big factors, though, which is both a very specific kind of team comp (and the Garen playing the same way/not making decisions to compensate for that comp) and the player not playing to Garen's strengths.
5.) [+1?] [X?] I don't really know if this is a good point or not. The whole point of my initial post is that I am not wholly convinced one way or the other, and I can't really decide whether this is actually a hindrance. My initial thought, though, was that lots of tanks don't have a gap closer.. not all tanks, but lots of them, and you're labeling this as a specific Garen weakness instead of player weakness. Meaning, since Garen doesn't have a gap closer, don't attempt to target a champion that would cause you to run through the crowd and take more damage than it's worth. It's not really a valid argument I think.
One of the things I considered about whether Garen's passive was too much was the way I like to play him. I generally stay in one lane and push turrets, keeping the jungle warded. By mid and late game, the enemy USUALLY NEEDS two champions at that turret in order to prevent at least minions from damaging it a little. If they only keep one, I can either poke them down enough to kill them (or make them back while I heal and pressure that turret) or at least keep sustain while my minions incrementally bleed the turret down. If they force TWO champions to defened? My team gets a free 4v3 team fight while they push another lane. Usually if more than two come to push me away from the turret, it opens my team up for serious split pushing, while I wait for the enemy to give up the chase, me heal, and then return to that lane to push. We win almost every time.
Now, I know I keep repeating myself, but it's important to me that it's made clear that this isn't an argument for his passive being overpowered. It just something that helped it be seriously considered and questioned to me. Maybe there's a better strategy for dealing with the harassment I described. We don't win 100% of the time, sometimes they have enough tank-busters or CC that only one champion is enough to prevent me from escaping when a second champion comes to help. It's just a very specific team composition that gives me trouble, and sometimes not always then if they don't use them right.