Well, that's always the trick, isn't it? If you can craft the definitions so that your position is true 'by definition', then it's 'true', right?
I'm trying to explain how the philosophy of naturalism infects all of science (both methodological and metaphysical) such that it can only return a 'natural' answer and excludes a supernatural explanation 'by definition'. This is making your position 'true' by definition and excluding all other answers by definition. That is a philosophical position, however, not an empirical one.
"I guess you could try to call this problem one of philosophy, but I just call it laziness."
You still aren't seeing it. It's not which model will prevail, it's that some naturalistic model will always prevail. Always. Always. This is due to the 'a priori' commitment to the philosophy of naturalism, nothing more.
"Ellis' model explained very well why we would see redshift due to gravitation. But where does Andromeda blueshifting fit into this?"?
Here you make the assumption that redshift/blueshift is a function of recession/approach velocity that is probably not correct. Halton Arp did a lot of work noting objects with physical connections and different redshifts until they took away his telescope for heresy. (reference back to Ellis' statements) Such is science.
Hey, if it works for you...
I'm trying to explain how the philosophy of naturalism infects all of science
I guess you haven't noticed that all of these terms include the word "naturalism." And it's a good idea. Even the highly-religious Francis Bacon knew it was a good idea because it frees you of the corruptive influences of what he called "idols." Rather than being an infection, it is a purification.
Here you make the assumption that redshift/blueshift is a function of recession/approach velocity that is probably not correct.
First of all, we know redshift and blueshift can be functions of relative speed and direction, just as we know the pitch of a train whistle gets higher at it approaches and lower as it goes away. It's the same phenomenon in both cases. However, we also know gravity can produce red shift in our views of stars.
The currently accepted model explains Andromeda's blueshift through approach. His model has everything with gravitational redshift, yet Andromeda is blue.