Sarang Noether

No, the opposite. There is a single list of commitments, and a single proof is used for all indices in the list corresponding to all signers at once.

Proof-of-concept implementation of spend and non-spend status: https://github.com/SarangNoether/skunkworks/tree/audit-proof

Yes, a Triptych proof can store 64 bytes of arbitrary data using a seeded PRNG.

The network can't detect it, so it's not possible to "remove" this feature. It's entirely up to client software to embed and/or recover this data.

This would almost certainly be separate functionality from the existing proof functions.

This issue specifically deals with the existing challenge construction, which is incomplete.

No worries. I'd prefer to keep it separate, to track the fix.

Addressed in https://github.com/monero-project/monero/pull/6329.

With Triptych, it is possible to hide up to 64 bytes of data in the proof by knowledge of a PRNG seed; however, any entity that does this can brute-force...

Since 2a and 2b share the same discrete logarithm across all points, they can be combined into a single proof.