Sharing serial output from one to many arduinos (plus a computer) via USB hub
First, read a bit about USB, it is an entire protocol, not just a hardware/pin based interface. It is actually quite involved. Then it will be clearer to you why a USB (from the Nano) solution will be... difficult.

Based on my understanding, I would use your #2 idea and what has been suggested, and do the following...


Setup the Uno as a USB slave, not a master.
Using the PC, connect to the Uno and the Trinkets.
When the PC gets data from the Uno have the PC "repeat" it to the Trinkets.


I think this will be the easiest and quickest solution.