I do agree with you on the point that I'd like to see some of the young players given a chance. I don't know the reason why that isn't happening at the moment (though Foden has been injured for a while now). It could be that they're deemed not quite ready or strong enough yet, but i think minutes could be found for them if we really want to regardless of whether Mahrez arrives or not.
Young players at elite clubs don't really tend to get loads of chances at 17 or 18 years old. Any examples you can give tend to be the exceptions that prove the rule. Woodburn and Brewster at Liverpool seem to be highly regarded but we accept players at "massive" clubs like that have to bide their time and often don't break into the first team til they're 20/21 (like Harry Kane). Being at City, training alongside world class players and under Pep is a great place to be at right now. People are always surprised when they see how old a lot of United academy graduates actually are e.g Lingard is 25 now. But it's because they have to wait for their chance and they seem willing to do so. Someone tweeted yesterday that United don't buy players when they have injuries and give youth a chance, citing Mourinho bringing McTominay into the squad when they had injuries. McTominay is 21. Foden is only 17.
We haven't really seen anyone leave our youth set up having not been given enough of a chance and lived to regret it so far. Sancho might turn out to be one that got away.
Regarding Mahrez's temperament, I don't really think he seems that moody. He gave them another year after they won the league and hasn't downed tools before even though it must be really frustrating he hasn't been able to move on to the next level. I think he would probably gladly accept fewer games if it means the chance to play in a team like ours and challenging for the biggest trophies. He wouldn't play week in week out at any elite club which is the sacrifice he will have to make to play at the very top. At the age of almost 27, he needs to make that leap now. He's a really good footballer, he might turn out to be unplayable in a team like ours.