I've translated a bunch of stuff in immersion from Spanish --> English, but I want to try it in reverse. Is one only afforded the opportunity to do this when doing the reverse tree? If not, how and where can I find articles to translate TO Spanish in immersion?
There are two ways of doing this.
You can add the course English from Spanish. And then go through the reverse process to get back. If I were you, I would add the course, then do a one word translation on a dozen articles that interest you, then go back to your normal settings for the site. That way, you can find your immersion articles in 'my edits' without having to go through all that nonsense.
Alternatively, and much easier, you can upload some articles in English yourself.
I think it's just a coding thing that they haven't got around to yet.
The cheat's way of keeping up with what's available to translate, if you're enthusiastic or have a bit of time on your hands, is to keep an eye on the stream of your friends who are native Spanish speakers and are working the other way around.