On 58a the Gemoro presents an answer of Rabba, then an answer of Reb Yochanan, then rejects the answer of Reb Yochanan and says "Ella Omar Reb Elozar...".
I presume that the "Ella..." is just rejecting the answer of Reb Yochanan, but the answer of Rabba is still valid. Do you agree?
Agreed. However, according to Rabah the Shevuah is not actually on the Shemirah; rather it is in order to collect his wage. According to Reb Elazar, on the other hand, if they don't swear they will actually be liable as a Shomer.