My question is: what is the obstacle to understanding atthi nāma along the lines of ‘is it really so?’
The father would be expressing their surprise that their son is eating yesterday’s leftover gruel.
FWIW, Buddhaghosa seemingly glossed atthi nāma as atthi nu kho.
It seems to me like a broken sentence, deliberately expressing the father’s reaction.
I don’t believe there is a precedent for using nāma as a question. In these cases it typically expresses surprise, outrage, etc. If it is to be a question, it is combined with an interrogative, as in kathaṁ hi nāma … “how on earth … ?”
Brahmali has:
“But, Sudinna, isn’t there … and you’re eating old porridge!
I suspect the commentary may be trying to normalize a quirky reading.