Why would Brewster be "putting out feelers" for where Decker would be drafted if he were to file? Brewster has absolutely zero incentive to facilitate Decker leaving for the draft early. Decker was drafted by MLB last year, but it was way late in the draft. He probably would be drafted again, a but higher. If he waits until his senior year, he will be drafted higher still, and would get a bigger salary and bonus.
One of the things about the baseball draft is that those who go pro right out of high school just spend more time at the lower ranks of the minor leagues, while those who play college ball spend less time in the minors. There really isn't all that much incentive to jump early for the pro draft in baseball. In football, sure, you get good money with an NFL contract, but a minor league contract pays little.