Hi there, fellow Chromecast user!
Let me try to answer your questions with my experiencies so far:
1. Chromecast is enabled by default with the UMS installation, you don't have to do anything
2. It appears once you use it, it doesn't appear automatically on startup.
The thing is that I might help you only a little, because I have an android phone and not an iPhone with iOS...
What I did was install
This version (5.0.0-a1) provided by SubJunk in
this post, fire up avia on my phone and I was ready to go, with out-of-the-box settings.
I also was able to play .mkv with .ssa and .ass subs, but that's more steps to explain.
The thing is that I don't know any app for iOS similar to avia. If you know any, try and tell me.
Hope you can make it work, tell me if you need any more help!