Since getting the ferric into solution is the main reason to add the oxalic, the first advice doesn't seem to make sense. Also, I have not heard from other sources that the ferri test was "unreliable" but I don't know the chemistry. I think there's a treatment of the testing procedure on Jeffrey Mathias' web site Pt printing guide:

Don't know about a nitrate test, never heard of one, but the chemistry is not my end of things.---Carl