Yes, originating cities never matter except in special cases (Great Zimbabwe, etc) where you're going to remember anyway. Yields are based on target city.
Yields are mostly from what districts are present in the target city. Length of trade routes don't inherently make a route more valuable. HOWEVER, over time you'll plop down trade routes in cities you've traded with.
If you do a really good job of creating a mini-game out of it, after a while you might be able to get a trade route that passes through multiple cities and trading posts. Since each trading post adds +1 gold, that helps a bit.
The other purpose of trading posts are to extend your trade route distance in the first place. I don't believe you need trading posts in your own cities to have your trade distance begin at the end of a road though. But of course they do add that +1 gold, still.
None of this info is really something we need a global screen to see, and in fact if you relied on a global screen you'd be doing a poor job at the mini-game anyway, since that requires careful geographical planning of routes to develop future routes.
