Sucks. I'm tired, and i guess i just wrote a miniguide for you to follow. instead of deleting it, i'll just post it.
1.) Yeah, can be. depends on how much of a difference you make.
2.) irrelevant. Lane =/= game. It doesn't justify a shitty mid game
3.) sick, so you could make something happen
Play a more dedicated champion pool. It will make it easier to carry these kinds of people.
Sounds like a case of division 5 syndrome. basically you win these games by pushing mistakes as hard as possible, and out roaming. In order to make this happen, you need to understand your champion and its patterns. While Janna can peel, you can't make that much of a difference in a game if your carries are potato.
I would shift to play a lot more blitzcrank, and pick up lulu as the enchanter instead of Janna. Simply because both of them make so much more of a difference in gameplay compared to janna. You want to be able to make picks and enable people on support. Since you can't rely on teams, pick someone out and make the game a 4v5.
Take these masteries on Lulu / Janna: http://prntscr.com/f2fgus
Masteries for blitz: http://prntscr.com/f2fi58
You benefit more from the safety of cunning and resolve than ferocity trees.
Lulu runes: http://prntscr.com/f2fh2f (mana regen quints / armor quints)
Janna runes: http://prntscr.com/f2fhf9
Blitz runes: Armor marks, seal of health, glyph of mr, armor quint, ms quint
Here's a checklist for support:
1.) At Least one ward per min
2.) Have a pink up constantly
3.) Ping a couples times every 5 mins to stop or direct allies. (Learn to shotcall.)
4.) Aim for the highest Kda in the game. no excuse to die more than once on janna.
For mid:
1.) Die less. Aim for 2-3 kills per death.
2.) Keep spamming similar champions (ahri / ori) to find your taste and make a pool. 2-3 champions
3.) Try for 80cs @10, and 180 @20
4.) try to roam 3 times between the 8-15 min marks