CHOIR
IF CHOIR - 0, ???RE is proved.
S??RE is impossible, as SHARE - 2. So the S and R cannot both be true.
Because of a lack of score change from STARE and SHARE, we know that both T and H cannot be the second letter*.
That leaves us with THORN. (N is disproved by knowing E, O is disproved by CHOIR - 0 assumption)
If the T were correct, ???R? would be incorrect. S???? would also be incorrect.
STARE - 2 shows us that both S and R cannot be incorrect (See the * above for disproving T; A is disproved by CHAIR - 0). So T???? is incorrect. Leaving us with THORN
@Mo: Check my logic?
EDIT: If CHOIR - 1, ??O?? is proved by the single letter change with score change from CHAIR --> CHOIR
Additionally, IF CHOIR - 1, S???? is proved. But I'll get to that after Joe scores it.