omg this is so untrue man
firstly nunu is about as op as it gets (on par with lulu)
Outside of laning phase. Within it, he's not super-amazing. He gets a multiplicative buff (though it lets you forgo/put off ASPD during laning) so the value of Blood Boil is low at the start of the game and ridiculous at the end of the game. Snowball is his harassment in an even lane and it can be dealt with. He offers no hard CC or sustain otherwise. His ult is fairly player skill dependent on getting the most out of.
Once a Nunu lane starts to win, he becomes a huge threat. Any item advantage is that much larger. He's able to zone that much better. It's that much harder to escape him (or his ADC) so getting caught out is that much worse against a winning Nunu lane. In a losing lane, he helps let the ADC keep up but doesn't get to do much else himself. He's melee range. He doesn't have much to do when forced passive besides Blood Boil and counter engage with his ult.
Come full item builds, Nunu with a full build ADC (gg 6-item Caitlyn) is insane. That's why he's picked. You could be forced to 2v1 the lane and Nunu with an end-game ADC is still worth it, honestly.
secondly, there are many diamonds who support only and go straight up the ladder
Quite a few of them don't play support at low elo ranges. Some don't even recommend trying to play just support to carry yourself.
I don't think the argument about relying on the ADC is a valid one because if you're the ADC you're just as reliant on the support. And don't do the whole support can't carry cause that just isn't true.
If you're the ADC, you're probably more reliant on the support. Creating the opening is harder than taking it. What I said was that a support won't carry a game, they'll create situations for the team to use. It doesn't matter how amazing of a support player you are if your team does nothing with what you do. That's what I mean when I say you're reliant on your team.
Granted, the same can be said about a lot of other roles. Like, a fed Kha'zix that picks off their ADC but the team still manages to die somehow due to engaging 2v4 or staggered fights or whatever. A good enough top, mid, ADC, etc. player can just have such a huge item advantage (+ mechanics advantage) that it's easier to single-handedly carry a game from one of those positions. A good enough Support will greatly increase your team's odds of winning in basically every team fight.
edit: maybe i should actually read the rest of your post before posting. I will concede that playing support at 1200 I can assume is pretty frustrating but once you get ~1700? maybe? there are many support mains and they occasionally single handily win the game. (zyra root, taric stun, nunu slow, janna tornado)
So basically my point? :x
I will note that low elo range supporting has the added advantage of dedicated support players being so rare that, if you're a good support, you'll basically guarantee bot lane wins before jungle involvement. Getting an ADC + dedicated support player is just that big of a difference over ADC + "I called top, gg don't respect call order I pick Tryndamere" Support.