× 帰宅すれば、必ずお風呂に入りなさい。Why is this incorrect? Is it because it is non-volitional? takes out the decision aspect?
ば only works as action and consequence. A common example given is "if you drop a ball, it will fall". You can't put a command, volition, etc, after ば, but rather a sequence of events; the result (ball falling) after the action (dropping the ball). And ば doesn't have a meaning of "when" either, and even translating as "if" would not make for natural English either at times. Like この通りを行けば病院につく. Again, action and consequence; go down this path and you will arrive at the hospital.
When you get home, be absolutely sure to take a bath.
A : 帰宅すれば、必ずお風呂に入りなさい。
B : 帰宅したら、必ずお風呂に入りなさい。
Jisho.org defines them as:
1. if so; in that case; in that situation
1. indicates supposition; if ... then; when; after
Ah, well I've bandaged it for the meantime but if that's awkward then it's fine to replace it with a plaster when you get home.