I'm posting again because 1. I'm on mobile and can't reply to myself for some reason... 2. While answering your questions, I was thinking about WHY this is so confusing at first...
Caution: This is just me thinking out loud, kinda, so just stop reading if you don't like it, and ignore me!
I think there are two things that throw people off in cases like this: 1. The Subject vs Topic relationship and 2. What an interrogative word really is.
For subject/topic: Imagine a kid's essay in English. "Cupcakes. I love cupcakes. My mom makes them. My dad eats them. I help eat and make them. Cupcakes are great."
Cupcakes were not actually the grammatical subject of any of those sentences until the last one. But they are still the topic the entire time. So you can see how subjects and topics relate.
Subject and object are all about clearing up what's going on with your verbs. Topic is more like the topic sentence of a paragraph to me. If I threw in some random sentence about bananas in my cupcake paragraph, I'd get scolded by my teacher. I'd have to transition to bananas first.
As for interrogative words: They are just filler words that request information. Pretend we're robots and you have to turn everything into a math statement.
"Who are you?" Change that to a statement. "You are who." Change it to what it really is: "You are x." (In cases like this, you use 누구)
"Who ate my lunch?" "Who ate my lunch." "X ate my lunch." (In cases like this, you'd use 누가)
And then our ? just requests that the listener input the value of x. You can do this for all of our interrogative words (though some more easily than others).
Once I thought of interrogative words in this way, it really helped me. I'd been taking their function for granted because I don't have to think about them in English, but I was having trouble as I startled learning Korean.
Anyway, ignore this if it's confusing. Just kinda wondering aloud, maybe it'll help, maybe not.
Particles can be rough at first, but remember all the trouble people have diagramming sentences in English? Particles definitely make that easier...