He doesn't "decide" the will is valid. He's not a judge. There was no option for him to say "no, the will is not valid". In fact, in the passage you quote, his assistant hands him a piece of paper and he says "oh, wow, yeah, not complex at all." He's stating the facts.
if "x" had not happened, is it likely that "y" would have happened anyway? If the answer is "yes," then "x" is not driving the story forward.
If the lawyer had not stated those facts, would Marta have not inherited the money? No--assuming the will is valid, she still would have inherited the money. The driver is the unveiling of the will.
The fact that the family can't accept that isn't deliberation. They whole point is that they have no power.
A decision would be "the family decides to go to court over the will." And then (next driver), "the judge weighs the evidence, and renders the verdict: the will is valid."
Or if you want to keep as close as possible to the scene, maybe the lawyer shows up having read the will already (somehow). As he sits there, he must decide: do I tell the Thromby's the truth, or do I decide to lie to them? You would have to show that deliberation somehow.
This isn't what's happening. There's no A or B here.