Syag, sorry but the 2nd condition you mentioned – that he be interested – is not among the halachos pf tochacha; it says to do so even if he screams at you “ad hacaah” until he hits you.
Regarding being qualified, you are correct – it’s also to be done without onaas devorim, “lo sisa alav chait” means don’t bear a sin because of him, that he is doing and that you could have stopped, snd don’t bear a sin yourself because of him i.e. the way you’re treating him.