^ I fail to see how 1 and 3 are plot holes. 2 might be kind of a stretch, but by my reasoning 1 and 3 make perfect sense.
Shinzon at least reached his teen years before he went into the mines. The Romulans were grooming him to impersonate Picard, which would presumably include knowledge of Starfleet regulations. Thus, Shinzon figures it is "safe" to put the droid parts on the planet with hostile tribal aliens, because it won't endanger Picard, because he knows regulations forbid Picard from joining an away team. He didn't count on Picard's recklessness, and was also maybe banking on the fact that he himself would send disposable soldiers, if he were in Picard's position, rather than going down to the planet personally.
Information on Data, rather than Starfleet Regulations (which is mostly common sense stuff anyway, especially the "Captain-stays-on-the-damn-ship" rule) would be highly restricted. He's an incredible piece of technology, one of a kind, distinct even from Lore and B4 in some ways. There's no way Shinzon or his team would know how to put together such a complex piece of machinery, especially in the damaged state they probably found it in. B4's reassembly probably took much more effort than the short collection of shots that the movie actually showed. Thus, they find B4 as a pile of severed limbs, and not a complete android.
Also, if B4 were complete, there is a slim chance that he might find a way off-planet, far away from where Shinzon guessed that Picard would eventually find him, or possibly get damaged beyond all repair by the natives. Even if Shinzon could have reassembled B4, doing so would not be a wise move. If you were going to leave somebody a note, would you leave it in a way that is sure to remain where you intend it to remain, or in a way that it could easily be lost, overlooked, or maybe even destroyed?