@about respectful titles

When we call the Pope "your Holiness", or the Queen "your Majesty", the verb gets third person endings, not second person endings —

"Your Majesty is considerate." (Not "are considerate".)

The same rule applies in Sanskrit —

zrNotu rAjA श्र्णोतु राजा "your Majesty should kindly listen"

Unlike in English, however, referring to a king with second person endings is not viewed as disrespectful, at least if some respectful address word is nearby —

zRNu rAjan शृणु राजन् "your Majesty should kindly listen"

Where does /pANini allow this?

IDK yet, sorry.

What if he didn't allow it nowhere?

Then, most likely, the commentarists noticed it and there must be a /vArttika somewhere. See, I'm not going to do all of your commentary-reading.