Hello @rusu83we have logged your save and will do some investigating.
To answer some of your above questions:
It's important to remember that 'concerned' is a 'pre-warning' just suggesting if it carries on there might be an issue. A player being 'concerned' is not 'unhappy' and just means there's something to keep an eye on. It's is VERY unlikely you'll ever have a top tier squad with everyone happy with playing time over a course of a season.
Some of this can factor in to a players attributes as well (I.e. more ambitious players want to play more)
1) Musiala is a star player. He expects to start every game & play pretty much every minute because he's the 'star player'. In your save he's being subbed off nearly every game, quite a few of the games with 20+ mins lefts, as a star player his concern is justified.
2) Abraham probably shouldn't be concerned, he's on the edge of satisfied/concerned but we'll take a look at this.
3) He wants to become an important player (enter contract negotiations and this is what he demands/wants) and, as he's close to dropping down from regular starter, needs these mins to keep on the level he's currently on.
4) Yup, but they're both being subbed off/missing games which is causing them to be concerned (it's just a concern though, they're not unhappy)
5) Goalkeepers are worked out differently as they're less likely to get subbed and more likely to be rested for cup games.
6) Player cares about playing time, the player relationship with a manager will impact how long before they get unhappy with the manager and complain to them/team mates and how the subsequent conversations go.
7) Individual players have a playing time (and a desire to play as much as possible) and aren't getting what they think is agreed. They might win awards in the future, but that doesn't affect the playing time they're not getting right now.