I suspect that non-trivial per impossibile counterfactuals, true subjunctive conditionals of the form "p→q", where p is impossible and the conditional is not simply said to be true on account of the falsity of p, are in a way like poetry: They tell us things that are hard to express in more ordinary language and that, moreover, have a deeper resonance with us, and are more plausible, when put poetically.
But we can, I think, give a sufficient condition for the truth of a counterpossible: if the material conditional "if p, then q" is true in virtue of a fact explanatorily prior to or independent of not-p, then p→q holds. This condition seems to me to also hold in the case of ordinary counterfactuals. Thus, the laws of nature are explanatorily prior to or independent of ordinary non-nomic facts. Thus, if it is a law of nature that if something is a raven, then it is black, we can say that if there were a raven in this room, it would be black, because the conditional "if something is a raven, then it is black"[note 1] is explanatorily prior to or independent of the absence of ravens from this room.
In particular, when the consequent of the material conditional is true and explanatorily prior to or independent of the antecedent, the subjunctive conditional holds trivially. For instance: "Were God not to have commanded respect to parents, there would (still) be a duty to respect parents." Here, the corresponding material conditional holds in virtue of the consequent's holding, and the consequent is (or so the asserter of the conditional claims) independent of or explanatorily prior to God's commanding respect to parents.
I don't know if the condition I have given is necessary for a conditional's truth. But at least sometimes, I think, we use a per impossibile counterfactual precisely to express a claim about explanatory priority or independence.
Here is a seemingly different sufficient condition for the subjunctive conditional p→q. If the material conditional "if p, then q" is more strongly necessary than not-p, then p→q holds. The idea of grades of necessity is perhaps best introduced by example: nomic necessity is stronger than practical necessity; metaphysical necessity is stronger than nomic necessity; narrowly logical (or conceptual?) necessity is stronger than metaphysical necessity.
We can combine the two conditions. Suppose that the material conditional "if p, then q" follows with a necessity of grade n1 from some fact F, and this fact F is (a) explanatorily prior to or independent of not-p, and (b) the truth of not-p is not necessary with a necessity of grade n1, then the subjunctive p→q holds. I don't know if this is a necessary condition for a subjunctive to hold. Maybe it is.