ElGamal scheme attack when one message is known and ciphertexts are intercepted
Knowing the message $m_j$ should not help you to get $r_j$. In-fact, ElGamal is assumed to be CPA secure (assuming that DDH is hard). Meaning that even if we choose two different plaintexts and give them to alice and she is encrypting only one of them, we won't be able to know which one she encrypted.

You can easily show that if you could retrieve $r_j$ ElGamal won't be CPA secure, as your can try both messages and try to retrieve $r_j$, you will only succeed when you try the right message.

Moreover, you are right about the messages Eve learns and how she does that.