b. recently due to new feature needs. core devs introduced a new sighash opcode that actually allows segwit tx's to malleate again..
Can you post some links proving that? That's going to be a big issue if it's really true. Plus, if it's true, how did the Core developers react when it was "discovered"?
there has been no change to the sighash type flags for a very long time. there still are the same 4 of them as always. by the way sighash types are not "op codes" they are more like flags.
https://github.com/bitcoin/bitcoin/blob/78dae8caccd82cfbfd76557f1fb7d7557c7b5edb/src/script/interpreter.h#L21-L28