At the outset, for the beginning of measure 51, I play the first three notes of the first beat in the right (A, D#, A) with the right (5,3,1) but then I play the fourth note in the treble (lower d#) with the left thumb, which my teacher suggested. Then finish the remaining 2 notes of the beat with the right (1,3) going back up. In that way, you avoid the awkwardness of crossing over.
My specific question is the fourth beat of measure 51, where the ppp is marked. Are you suggesting at the ppp that the fingering with the right hand would be (at the last beat of measure 51, at the ppp) 5,3,1,5,3,1 going down the keyboard and then 2,3,5, 1,2,3,5 going back up?