"Possibile & aequissimum erat" is an impersonal sentence, which can be translated in English as "it was possible and most fair", but which needs no such pronoun in Latin.
What was 'possible and most fair' is here expressed as an accusative-infinitive, with quam (indeed referring back to legem) and praestari.
So "... legem posuit, quam praestari ... possibile ... & aequissimum erat" is literally: *"... He laid down a law, which it was possible and most equitable to be fulfilled". Which is to say: It [impersonal] was possible and most equitable that it [the law] be fulfilled.
However, in English, we can't use an acc-inf with an impersonal sentence. "I want the law to be fulfilled" is correct, but *"It is possible the law to be fulfilled" is not. We can insert 'for' before 'the law' ("It is possible for the law to be fulfilled") or we can use a that-clause ("It is possible that the law be fulfilled"), or perhaps both "It is possible for the law that it be fulfilled" (?). But for some reason these solutions look strange in a relative clause: "A law, (for) which it is possible that it be fulfilled" (?) or "A law, (for) which it is possible to be fulfilled" (?).
Substituting the impersonal subject is also wrong: both *"The law was possible to be fulfilled." and *"The law, which was possible to be fulfilled."
So I'm afraid the best way to handle this sentence is to turn the passive into an active and turn the accusative from the subject of an acc-inf clause into a simple object of the infinitive: "... He laid down a law, which it was possible and most equitable to fulfill" (as if the Latin said "legem posuit, quam praestare ... possibile ... & aequissimum erat").