Sorry I am a few months late, I love reading old forum posts and his one had caught my attention. So first thing to note is that your MMR greatly impacts where you place after placements, even though they don't say it, your season MMR and your preseason MMR both factor into your post promos elo. However, the system does not look at just you end of season MMR, it looks at the season_as a whole_. Here is the formula I think they use: EndSeasonMMR+PreSeasonMMR/BeginningNewSeasonMMR X PlacentGamesMMR This is supposed to be a reflective start zone, but this isn't always the case; those numbers are predetermined by the system, and pulls the number coordinating to your placement matches, and how well you performed in said placement matches. 10 games gives you an MMR multiplier to be used at the end of your coordination set, this isn't always a whole number its usualy less than 1. And that's total bs, but that's not all.
If this was your friends first season playing ranked, they will be placed above you. On my smurf account, (the one posting this) I went 7/3 in placements, and was in B1 last season, because of the formula, even though I carried, I was placed into B5. My first season friend, went 4/6, and got Silver 3. What flames, and trolls don't understand, is that your season career, sticks with you for almost 2 years. But, there is always dawn after darkness.
Your personal MMR is being compared to your elo and division's MMR at end game, so if you have a gold MMR playing in Bronze, then if you win and have gold performance you will earn more LP than usual, keep in mind the system is designed to reward underdogs and will not give as much LP to those who were along for the ride. But, the reward is minimal, this is why we have solo queue back.
I hope this helped someone, and if any of the information I have proposed is incorrect, please correct me, I am only human