It's not about babysitting. God, I fucking hate this term.
Is the lane gank-able and would it benefit your team to gank? Yes. FUCKING GANK. Ganking once is NOT babysitting. Ganking twice is NOT babysitting. Ganking three times is NOT babysitting. It simply means the opponent is a fucking retard and can't ward/control his lane properly.
And didn't you just say he gave firstblood at lv2 to a XinZhao gank? You said in your first post he was level 2. This means you're level 3. Especially when you're playing a very fast-clearer like Nocturne.
This is why everyone's all like "ermagerd my teams suck i cant carry blablabla"; well, you can. You SHOULD have gone up top and helped him. This is what soloqueue is about. Carrying your team through lanephase and then hope to god you've either snowballed the game too hard in your favour, or they'll listen to your calls. (This isn't aimed at you directly, by the way).
The point is: YOU COULD have prevented his death by counter-ganking Xin early, you saw he was pushing to his turret, should be obvious tell that Xin is going to gank (I would of). Or you could at the very least have scored one, or possibly two kills off of that gank. You'll be level 3 by 2:40 as Nocturne. Easily.
Anyways, it sucks the Teemo left the game indeed, I feel for you. But he was obviously in trouble and you should of focused on his lane, or let him know that the lane was beyond saving, so you're going to focus on other lanes instead, and to just play passive and wait for lanephase to end. Yes Teemo sucks if not played right, and every Teemo player suck at playing Teemo. Except enemy Teemos, ofcourse!
And Firebert; for the love of all that is fucking holy. Stop with AMR. It is beyond idiotic.
And yes, Teemo will derp all over Garen if played correctly. Garen pops Q? Well pop W and move away. Garen takes a CS? Hit him once or twice. The only the this guy should reach you as Teemo is if you fucked up, he flashed or their jungler ganked. IF however you let Garen near you, he will completely destroy you.