Hello
I have some questions about this exercise.
I already mentioned that this book by Persichetti is like a “bible” to me. I also do the exercises he suggests.
In this case (I have the Spanish version, but I think it's well translated), I think it says that the added notes are on the cello and double bass (in octaves). Strange, isn't it?
I've only analysed the first few bars, but I think your added notes are more in the upper registers, is that right?
Regardless of all that... it doesn't matter. The piece is very beautiful. It sounds emotional and intimate, and everything fits together very well. Perhaps it's a very heterophonic style (I mean, with little counterpoint), which I don't think is bad. It reminds me a little of some of Mahler's work.
Very good work.