For leveling, Discipline is better due to its durability.
For solo content and farming old raids, Holy is really the way to go because its the only spec with decent AE damage. In old raids, your single target damage will exceed your need for virtually any encounter. In PvE solo content, normally you'll either be fighting large groups or single targets alongside all the other people doing the same world quest (where your ability to survive matters quite a bit, your ability to dps not so much). I suppose, worst comes to worst, you'll have to fight it out alone. But winning a fight 30 secs after a Discipline/Shadow Priest would have won the fight isn't a big deal when it's a single fight that it took you 10 minutes of travel time to reach.
I can't think of any. Discipline's weak reactive healing and lack of available Atonement targets really cripple it in 5-man. It's entirely possible there exists some oddball boss I can't think of off the top of my head, but I can't think of any 5-man situation where Discipline is a better choice than Holy.2) Is it possible to require switch to discipline for a specific dungeon or boss fight (mythic dungeon) because it will be situational much better than Holy?
The trick of selling a FFA-PvP MMO is creating the illusion among gankers that they are respectable fighters while protecting them from respectable fights, as their less skilled half would be massacred and quit instead of “HTFU” as they claim.
If you're struggling in a mythic+, it is very likely your group is not doing what they are supposed to. And remember; CC stands for Crowd Control, not Constant Cleave like my guildies want to tell me...