As a native speaker... why not "Miluješ ji?"?
Idk, it's broken man... I did that too :D Taky jsem Čech lol
How would you explain things like "I love you, but I am not in love with you." if you do not try to enforce the difference that exists in both languages? There is more to making a course than playing the native card.
To by bylo: Do you love her?
Nice to see that "zamilovaná" works, too :)
Somehow I had the notion that putting "jsi" as the first word of the sentence is considered slang or impolite. Am I wrong?
Yes, you are. It is completely normal if you use ty to address that person. If you use vy to address that person, you say "Jste...".