If you have Salzedo’s markings, they compensate for acoustical shortcomings on most harps. My harp is now so full-bodied they are not necessary, but on younger harps they are.
So now I have to count 165 bars? I don’t see any bar in which your question applies. Moving the octaves in the bass an octave lower engages the harp more fully so it has the oomph it needs. So yes, unless you are playing on an old Erard.
I would say the molto slargando does not apply to the arpeggio, but to the following bass D and the octaves melody following. I think that a few of the tempo indications in the piece are unclear or missing. I have heard the piece played on 23s with and without Salzedo’s editings, and his editings definitely enhance the drama and excitement by causing the harp to resonate more fully. It helps avoid having people overpull a single string trying to get maximum volume out of it. Again, this piece has certain demands that most harps cannot respond to fully enough, until they are very mature, so the doublings are needed. Someday, we will have the Salzedo Edition published.