Well..I read the tldr...
It sounds like you think she is in a bad spot though, and I totally agree with that. When people are 'forced' into support and pick Janna, I think 'well that's why you hate supporting'. When people first or second pick it and want to support her I don't even know what to think. Her lane presence is zil. She typically hides even farther back than I do (assuming I'm adc, that's when you notice it most) and just does nothing. You'll never snag a kill with janna support unless they dive turret or screw up, without a third party (whereas mf taric or something occasionally can or blitz anyone).
All she can do is give you a chance of escaping when they all in and destroy you. With any luck shield + nado lets you escape with your life (but not your game). I'd rather see a Janna go mid because nado is good at pushing and maybe she can contribute with roam. When I see her bottom I just sigh. And when the other team picks lulu or taric or someone good... Taric sitting in a bush makes janna an exp sponge, and lulu or sona are just so much better. Their cc is better, their damage is better, they have a presence in lane.
I usually rush Athene UG and CD boots, I guess I have to say.
I really don't notice her being underpowered, squishy at the start yes, but I build supporty/aura items.
So you rush grail, get cd boots, and also get support items? Jesus christ you must be getting pentas.