Regarding His saying, the Exalted: “...so let them be chaste...” Is the lam here for obligation?

The second verse is not about the foolish; the second verse is about orphans. The orphan is tested, but the woman is not tested in the same way by the Shari'ah, because she is not of that category. She is tested by the needs of the household. If she is known to be capable of managing the needs of the household, then she is prudent. If she is known to be foolish—for example, if she kneads the dough and gives it to the cow, or if she makes the tea and pours it on the floor—then she is not considered capable.