Could 魚が食べる change in context? Marking 魚 with を would grammatically make 魚 the object of the action such that it would mean "Fish are eaten"; if 魚 is marked with が, 魚が食べる would mean "the fish eat", if I am not mistaken. However, depending on the context of the conversation, perhaps the topic being あたし, could 魚が食べる mean "the fish are eaten", in a such sentence as あたしは魚が食べる?Would that thus mean あたしは魚が食べる would mean "I eat the fish"?
You seem to be confused about how the を particle works. 魚を食べる does not mean "fish are eaten". It means "(some unspecified person) eats fish".
To say "fish are eaten" you would have to modify the verb to the passive form 食べられる and the object would become either the subject or the topic, but that's a separate discussion.
You are correct that 魚が食べる means "fish eat". But your sentence あたしは魚が食べる is pretty much meaningless. You are establishing yourself as the topic of the conversation (あたしは) and then going on to say something completely unrelated to you (fish eat). However, with 魚 as the object you get a perfectly reasonable sentence あたしは魚を食べる (I eat fish).
In answer to your title question, が always marks the subject of the verb. Sometimes, as in あたしは魚を食べる, the subject is promoted to the topic and when it is clear from context (as it is in this case) it is omitted.
Things get more cloudy when you consider sentences like 私は魚が食べたい (I want to eat fish). 魚 is still the subject in this sentence even though it seems like an object in the English translation. This is just due to an inability to translate 食べたい into natural English. You can think of it as "For me, fish are eat inducing", and then 魚 remains the subject.
As to your final question "Could 魚が食べる change in context?", I'm not really sure what you mean by this. Potentially yes, but I can't guess at what kind of scenario you are imagining. You'd have to give a concrete example and I'm afraid the one you tried to give didn't really make sense.