While possible, there are some descrepancies. If the Orz are indeed an extention of the Precursor, then why is there a problem with the translation. It is clear that through dealings with the Slylandro that the Precursors were at least able to manage common dialect that is obviously maintained well enough so that you can make out all of what the Slylandro tell (though I will give that they have also had interaction with the Milue and the Melnorme). As a lesser extension of the Precursor, the Orz should at least be able to manage something like that. Also, the thought of sentience within sentience does not seem plausible. If a portion of sentience within another sentient being tried to rebel (felt imprisoned or whatever), it could mean the end of the greater sentient being. It'd be like a person's liver rebelling against an alcoholic. I don't think that a sentient being, such as the precursors (with as much power as we assume they have) would give lesser portions of themselves such possibly self-destructive power. I also do not feel that the Precursors would just allow a part of themselves to remain in real space, a place which they undertook such great feats such as the rainbow worlds to leave.
Hmmm....
I think I can answer most of that. The fingers evoled sentience seperate from the Precusors. Thus, they may have developed their own language and hence the problems with translation. Senteience within sentience has been done in other sci-fi stories. An example of this is Vampire Hunter D. D's hand is itself a seperate sentient being. Most sentients that are part of another sentient would probably not rebel as we can assume they have evolved enough to know that destroying the host will most likely destroy themselves. Even at the earliest stages of evolution some form of instinct or genetic programming would likely prevent this. Finally, I believe the Precursors may have allowed the Orz to remain behind so nobody would get too suspicous and go snooping around for the Androsynth. They figured most races would simply think that the Androsynth were conquered by the Orz and leave it at that. The Orz had never been to *heavy space* so the Precursors might have just made a concession in order to keep them happy and/or occupied so that they could get on with more important matters.