But none of those scenarios involve any real platoons. Hoskins and Castellanos still start every game (except for normal rest/off-days) one way or another. Hall only gets to play - or even be on the roster - while Harper is out, regardless of whether he or Hoskins play 1B. Against LHP - the less frequent situation - it would be Vierling RF, Castellanos DH, Hoskins 1B.
If Harper's only out for a month or two the above is exactly what will happen. But if they think he's out 2-3 months, or that he can't play the field (which I don't think will the case, once he's rehabbed, he's rehabbed) they could use a LH batter who is better - and able to play the OF - over Hall.
And the way they "used" Hall down the stretch and in the playoffs suggests they might already think that. It wasn't just that there was no role for him off the bench. He had a very impressive stint over 136 ABs but you put him at 1B and he probably becomes a (slightly) negative WAR player. And if he hits a sophomore slump or adjustment period at the plate he won't be a good enough DH either (though neither was Castellanos obviously).
Basically they need 2019 Brad Miller.