Greenfield, I know you're a very tenacious discussant (is that a word?), but in this instance, I believe you're arguing an interpretation of rules text that is not the only interpretation one could come up with.
"Class level The character’s druid level. The druid’s class levels stack with levels of any other classes that are entitled to an animal companion for the purpose of determining the companion’s abilities and the alternative lists available to the character."
"Levels of any other classes that are entitled to an animal companion" - doesn't say "class levels in any other class with the animal companion class feature". The relative pronoun "that" could easily refer to "levels", not "classes". Ranger levels are entitled to an animal companion, at half their actual number. "A Ranger's levels for the purpose of attracting an animal companion" equals half his class levels. I think your insistence that the sentence above talks about class levels, instead of "animal companion-entitled levels" has led you astray.
Also, your insistence that the FAQ doesn't provide a base for argument is groundless. It has been put out as an effort to help players interpret the actual rules text. It's like a precedent case, where the rules text is the law. Both taken together is what a judge (the DM) must reference in order to come up with a just ruling. That, in certain cases, precedent might have been contradictory to the law in a few instances doesn't change the fact that it provides one resource among others to draw on when one is trying to come up with a satisfactory, just verdict.
The FAQ may have been wrong before, but you won't deny that in this case it seems to be on the right track where RAI is concerned. Now all you need to do is apply it as a guideline to help you interpret what the rules text actually says. It's not as plain as day, obviously, or we wouldn't be having this argument.