Cute puzzle. 3 operas are enough, as there are only 20 variants. A first try with 2 singers on each side splits the possibilities as 8 (neutral), and 6 each in case of success and failure. 8 possibilities can be told apart in two tries, by putting one now known star one one side with one of the original pairs, and a singer frmo the other pair on another side. This splits the space into 2-3-3; the third step is straightforward.
The 6 possibilities in cases of success of failure can also be distinguished in two tries, first by dropping one singer from one pair, to yield the split 2-3-3, etc.

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