How to get high elo/MMR from placement games?

So this is less educational but more of a story with ? at the end.

I had a friend that played League back in 2014 he played ranked and everything and got place Silver 4, I don't know about his KDA or cs per min etc but he wasn't a god like he wasn't a diamond player in the ruff, he was just "decent".
He and I would play with our other friends all scaling from Diamonds all the way to Silver with some Bronze players also but only in normal games, arams and customs no ranked.

He then stopped playing league for the most part, he maybe played about 100-150 total games normal games but 0 ranked. With us in again normals with diamond players - bronze. He came back since missing the whole 2015 year of ranked so it's now 2016 he played ranked where he said "win or lose, I'm going to focus on me".
He was putting up 6.0-7.0 cs per min, stalled out games to the 40-50 min mark so he could have 200+ cs per game and only worried about his KDA. He only won 3 out of 10 placement games and got placed Gold but his KDA was around the 3.0-3.5 around that range.

My questions are

1. If you play ranked say 2017 and get place say Silver 3 but you don't play ranked for a full year the next year but instead just normals with high elo players(plat-gold etc) does that trick the system to think that you're a high elo player but you got "unlucky" only winning 3 or so games or what?

2. CS per min and total cs per game is the main MMR in your placement games to see where you really should be placed? Had multiple friends who only had a 3.5-4.9 cs per min because they wanted to help their team but got placed low bronze or low silver but had other friends who only focused on cs and nothing else and they got placed Gold 3-4 where 2 years ago they were only bronze 4-5 / Silver 4-5 themselves. Both groups of friends either placed in Gold or Bronze had about the same KDA.

Overall I think worrying about cs per min and per game as well as clearing wards more so than just laying them down + KDA but not so much about winning is the main MMR/placements that decides what elo you belong in.

• Thank you and Have a good one!