"Free from doctrinal error" means that the opinion so labeled doesn't contradict authoritative Church teaching. But if the Church has not taught authoritatively about something (the fate of unbaptized babies, for instance), it is difficult to offer an opinion that would contradict a non-existent teaching.
Thus, on a given subject where no authoritative teaching is given, a diverse set of opinions, even in contradiction to each other, could all receive the mark "Nihil Obstat."
In other words, what you are telling me is that Rome can declare two completely oppositional Teachings to BOTH be "Nihil Obstat": completely free from all Doctrinal Error.
Apparently, in order to Defend her proclaimed Infallibility, Rome is willing to appeal to the Insanity Defense.
Sink, I do wish you well. I owe you this one... you've given me my best laugh of this fine Lord's Day.
I guess this is what Romanism comes down to.... well, at least it's hilarious.
That's a virtue in itself.
Best to you and yours, and may God bless.
Regards, OP