That is a very nice explanation. The process for a mortal to become a god could be a huge ritual and all that - takes years/decades, masses of people praying, maybe some hideous sacrifices if going for the dark god thing, etc..
As for people becoming followers, I believe something can be put together where an individual can
convert to the realm of channeling is in order. Conversion is a big part of religion, isn't it? A DP cost depending on the type of spell-user, with the possible loss of some (or all) existing spell casting capabilities as a result, could be in order. In effect, you are switching teams.
Why someone would do this could be any number of reasons: the new god grants powers that help dealing with a particular enemy, has a great dental plan, - plus many promise a particular paradise for their followers in the afterlife - who knows.
This could create some interesting campaigns dealing with the new "upstart" religions/gods conflicting with the established orders, I believe.
If you think about it, this has to be possible, as the very first gods had to convert some people because why would the first people have their base realm as channeling?!? Unless, of course, it was the gods that created the people in the first place - then why whould anyone have any
other realm than channeling?!? Of course, in a massively varied game-world both could be true
Yes, this sort of breaks down to the "chicken or the egg" argument (though there really isn't an argument: something chicken-like gave birth to the actual chicken).