[to Riot] ranked promotions statistics

Hey Riot, I have a few questions regarding statistics with in-bracket (e.g. silver 2-to-silver 1) promotional series in ranked leagues:

1. How frequently do people, on average, successfully complete a promotional series?
2. Does the bracket (bronze, silver, etc.) affect how frequently people are successful in their series?
3. Over the course of time, do most people eventually complete any series for which they have at some point qualified? Meaning that if Joe Schmoe qualifies for s3-s2 series, will he likely eventually get it even if he had to replay the series 4 or 5 times to do it?

I am just trying to figure out reasoning behind series since they seem to be really random. All it takes is one troll teammate out of the eight you need on your two winning teams to lose a series, so I'm wondering if series are actually important measures anyway? Maybe it would be better to just set LP/bracket at 125 points and remove the best-of-three series altogether.

Thank you for your time.