Why does the Qui-gon voice need to be in there at all, or the entire part where yoda tells Obi-Wan he has training for him on Tatooine? Before the PT I had no problem believing that Obi-Wan knew how to come back as a force ghost. I figured it was something he learned to do after he was killed. Why even bother with trying to explain it in the PT?

All you would have to do to achieve this to cut the scene of Yoda meditating, and Bail informing him Obi-Wan is arriving.
And later on the Tantive 4, after Bail stands up in the conference room on the Tantive 4 and Yoda says "and now into hiding we must go" cut to Bail walking up to Antilles to give the droids to him.

There is no need to explain anything about the force ghost or mention Qui-gon. It dumbs down the Force for the audience, and makes it less mysterious. Let the audience draw their own conclusion as to why Jedi can come back as force ghosts.

This would remove the need to insert and mix dialogue from other films that does not sound good and stands out very badly.