i'm of course not a pro, but i can maybe answer some of your questions a bit.
Q: Why do casters look down on Guinsoo's Rage Blade so much?
A: you get pickax then then you have to wait for about 2k gold. during that time, you can't really do much, but it's not a bad build for trist, though it is a bit less common. it helps trist's mid game and her late game is still really strong. from hearing them, they aren't actually looking down at it, they were fairly hyped on it really.
Q: Why aren't supports building Zeke's anymore?
A: other support items are just being prioritized a bit more right now. it can also depend on the games and situation. i don't know too many details on this though.
Q: Why do I never see Ezrael build a Trinity Force and I never see a Corki build a Frozen Gauntlet?
A: with ezreal, you want better kiting since his get away is up more often and he can use Q to poke more often than corki can. corki wants the damage from triforce more to burst people down.
Q: Why didn't Mao Kai get picked in TSM vs. CLG
A: they could have, but they just wanted other picks instead, like grag.
Q: I kind of wish Bjergeson would have picked Zed. Was there a good reason not to in game 5?
A: watching games now, so not too sure of details right now. looking at the comp on the other team, picking an assassin would be hard vs all the cc and peel. the main target would be trist, and she had massive amount of peel.
Q: Will we ever see Volibear in pros?
A: if voli is buffed maybe, but as he is, there are better picks for what the teams want. poppy gives a lot of cc, it's not just about damage. he is also easier to kite than poppy or other picks since he has to actually run at the team to get anyone.
Q: If this is the tank meta and nobody is banning Vayne... why isn't Vayne getting picked or played?
A: vayne isn't strong right now, even with the meta as it is. other adc are just dealing more damage most of the game than vayne would (it would have to get to late game and if they picked vayne, that would become really hard since they couldn't really win any teamfights for a long time).
Q: Is Ryze really strong if you are not Faker? I haven't seen much success with him outside of Korea.
A: not too sure on this.