"We saw the best of Ian Happ; Schwarber has MVP ceiling." IOW, his rookie production is what Happ is going to be. That's not believable. Every other prospect the Cubs have brought up were supposedly going to lower their K rates, improve BB rate, etc. But Happ can't? So Theo made a mistake drafting the best hitter in D1 baseball? And of course Schwarber has an MVP ceiling and other clubs believe that same thing, but Happ is more tradeable than Schwarber? Weird.
This is what was said about Happ draft-
http://m.mlb.com/news/article/129367126/cubs-select-college-outfielder-ian-happ-at-no-9/
Think your quote from McLeod says it all. They thought he was a 15 HR hitter and he hit 33(if you include AAA) which should tell you why Sahadev Sharma thinks it would be selling high. Can Happ lower his K rate? Certainly. But that's isn't really going to make him all that more valuable. And any team acquiring him will assume like an young player those numbers come down. As for why you deal him, do you really think there's more power in his bat? If the answer is no then that's selling high. Outside of waiting for him to be an all-star player(if he even gets there) his value isn't going much higher.
As for Schwarber, he hit 30 HRs in a year where he hit .211. Without stretching much it's not difficult to see him routinely be a 40+ HR guy. His peak years may be 50+. What gets lost in Schwarber's struggle was what he did in the minors. For reference here Happ was a .275/.362/.472 hitter in the minors. So, seeing him hit .253/.328/.514... there's a little room to grow on what you'd expect him to be but not a ton. Schwarber on the other hand hit .334/.432/.619 in the minors. He was a complete hitter in the minors. He hasn't been that so far in the majors. I'll leave the "why" to other people but if you hit .330 in any league you're a great hitter. Schwarber in the majors hasn't even approached good. And the other thing is he wasn't even a big K threat in the minors. He struck out 141 times in 665 PAs or a 21.2% k rate. So clearly there is a vast difference between the player Schwarber was in the minors and what he has been in the majors. Does he get better? Who knows but you can see sparks of what he can be. He's been a monster in playoff baseball where you see the best of the best. Just needs to do that more consistently.
As for why Happ is more tradable, think that is pretty obvious. You have both him and Baez at 2B. I've made the case as to why you would deal Baez but to put it bluntly if Happ has anywhere near the value Baez does trade wise I'd probably deal Happ first because however I feel about Baez it's clear you can't just find people who have his defense. As for CF for Happ, he's not amazing there and when he did play there they moved him as soon as was feasible. That leaves LF which is Schwarber vs Happ. That's a case of selling low on Schwarber or selling at near the peak for Happ.