Better to play on your own account, so that your MMR match your skill level and not your friend's, though over time I guess it won't matter.
As long as you believe in the so called "elo hell", you're sure to never leave it. People who are in "elo hell" are there because that's the rank they deserve. Sure, you can blame some of your games on bad teammates and trolls, however in the bigger picture these matches account for so little that they don't really matter. Forget about your teammates, look at mistakes you make and how you can improve. That's the key to getting better.
Just find a role you like, main a few champs in that role and make sure you know how to play atleast 1 champion in the other roles. One of the reason some people never leave "elo hell" is because they are simply "filler", with no real specialization.
From personal experience, Support maining is really good for ranking up on low elo. Get your marksman fed, and you will win most of your games. This works really well from Bronze to Plat, and all my friends who are Gold or Plat are support mains.
Also, if you're not yet using Smart Cast, start using it. It's difficult in the beginning, but a few games is all you need to get used to it.