[chryuop]

Yes your reasoning works fine. Yes the phrases are correct. Yes the phrase changes if you put wa or ga. No, it is not that easy to explain it hee hee.
I guess for most of the Japanese language students the difference between wa and ga represents the first and biggest problem.
sakana wa taberu and sakana ga taberu both mean the fish eats, but in Japanese they have a certain difference. Sometimes to help us understand (us as beginners) they tell us to translate the wa with "as per..." or "for what concerns the...", but it is not always easy to see the difference.
The way I see it is that when you put the wa, like in my example, you care about the fish and the rest is additional information. While with a ga you main concern might have been something else. I don't know, maybe we were talking about the sea (as topic, my main concern) and the fish eats is just an additional information of the sea.
Hard to explain...only using it will help you understand it.