I think your teacher is fine player, but I just listened to the first mvt. with score and cadenzas in hand; I don't see what you mean, he plays the Cadenza #3 to the first mvt, with a couple of extra beats.; do you mean one of the littlle cadenzas in mvt. 3? Am I mising something here? Maybe I'm tired...
When I played this 6 years ago I did the first cadenza, where Beethoven invites/challenges the pianist to (Fortsetzung fehlt) finish the cadenza from where he leaves off; I added 16 bars to meet his challenge; it worked out quite well, I would post the recording but the sound quality is very poor and the orchestra is even worse...they are better now....