Pretending for a second Mercer and Holtz are at the same point in their respective progression arc, I feel like Holtz would be the only one "hurt" by being rushed. His game seems like a top 6 or nothing game, if you shove him down on the 3rd or 4th because he's not ready for top line minutes, he will have to learn to play a different type of game while ignoring the skills that he should be working on. Mercer is more like a Coleman, put him anywhere and he will get shit done. He seems like the type of guy who can work his way up the line up and change his game along the way. He might not have the ceiling Holtz does, but his style plays better when tossed around the line up.
With how the lineup is currently set up the 3rd line could have Kuokkanen and one of Mercer/Zacha. I don't why people still think in terms of the archaic top 6 and bottom 6. 3rd lines aren't devoid of talent anymore. That lines probably doesn't work with Zacha at center because no one there is driving the bus. If it's Mercer and he's a capable NHL center then it's a perfectly fine place for Holtz to be