Maybe it is not correct to still call it Mentor Control, but Jeskai or 4c builds that are based on cantrips, Delve-spells and Planeswalkers are still pretty strong, so I place my questions here.
Why do people not play the full set of moxen? The deck is loaded with spells which can profit from the extra mana (Vryn Prodigy, Snapcaster, all the Planeswalkers, the win conditions, the multi artifact removal you bring out of the board, the two mana Dredge hate ...). While watching streams with these kind of decks I noticed that people often draw many cards but are forced to discard at the end of turn instead of just casting the spells because they are missing the extra mana and tempo boost of extra Moxen. In my eyes, cutting the the off color Moxen only helps in the Xerox mirror (being either Delver or Control). With all Moxen you play 22 mana sources overall before boarding. That still leaves you in a favored position against other blue control decks like Grixis Thiefs or Oath that play two or three more mana sources. And against the rest of the relevant field (Shops, Dredge, Paradoxical, BUG) you even want to have the extra mana and tempo boost.
Why do people play four colors, in most cases UWRG? G is usually added to add Sylvan Library and Ancient Grudge. But this weakens the mana base a lot and Strip Mine (a nice tactical option) gets cut. Even though, both cards are great, I don't see the reason to add them as you have plenty alternatives in the Jeskai colors. For Sylvan Library you can play additional Vryns/Snapcaster/Merchant Scroll if you want. And it's not that this deck isn't already drawing lots of cards. And By Force is a powerful tool against Shops too. Even though Ancient Grudge is an Instant I am not sure if it is even better (you have to play twice for Sphere effects, you have to fetch for serval different Duals, you can't remove more than two artifacts). With just UWR you can play three different types of basics and give Shops a hard fight on that route.
Thanks in advance for some more information on these points!