I tend to agree that Castellanos wouldn't be the best use of resources, but also I don't think the logjam is as problematic as you imply here. When everyone is healthy:
Adam Engel optioned to Charlotte
Starting OF of Eloy, Robert, Castellanos.
Mazara takes a 2-3 games a week from Castellanos and Eloy, mostly against RHP. He takes another game a week in RF when you shift Castellanos to DH.
EE at DH 4-5 games a week, Castellanos and Grandal/McCann when you rest him (at his age, we probably don't want to push EE for 140+ games anyway)
You also can move people around to give Jose one game off a week.
That is when everyone is healthy. As soon as one of Robert/Eloy/Castellanos/EE/Jose gets an injury (which seems likely), you shift it all around.
The only disadvantage is you are losing Adam as a late-inning defensive replacement. In a really healthy season, maybe his defense is worth more wins. But if any of those 5 guys misses significant time (or if someone like EE hits the age cliff) having both Castellanos and Mazara makes you a MUCH deeper team, and would obviously be worth more wins.