The information was too sensitive to be passed through directly to Trump but then Don Jr uses his private email system can be archived and then retrieved by government sources. The only explanation I can think of is that Don Jr wouldn’t know the way they usually talked to Russian officials which wouldn’t make sense in my mind because he had such a high role in the campaign.
This is a bit harder to answer because we don’t know how/why the information was retrieved, so I’m just arguing from what I believe. Yes it would have been used maliciously against a political opponent but it most likely wasn’t retrieved by intelligence agencies in Russia so they can use it against Clinton in her electoral battles considering they had a plateau of reasons to collect info on Clinton including the fact she was a secretary of state, state senator, first lady and this was all in between 2000 and 2014 when she announced she was running, this information could be years upon years old.
I can’t answer that one other than referring to my previous points, I feel like this was the only actual channel of communication when it came to this (the Donald Trump Jr. emailing Goldstone) but Mueller will most likely go through Manafort and Kushner’s email following this, I guess its just a wait and see thing as is common with a developing story.