Wasn't the Hawaiian Legation excluded from testimony?

Yes, the Hawaiian Legation (i.e., the United States governmental representatives in Hawaii, such as Minister Willis), was not compelled to testify. Neither were the diplomats from the Provisional Government of Hawaii.

However, this was not done to suppress adverse testimony. In fact, the entire Blount Report, and Blount himself, were presented at the hearings - both of which were highly critical of the Provisional Government and Minister Stevens. The reason for excluding the diplomatic corps of the U.S. to Hawaii and Hawaii to the U.S. was because negotiations were still ongoing.

It was not thought to be proper to question the diplomatic authorities of either government on matters that are, or have been, the subject of negotiation between them...

It would hardly have been appropriate to compel testimony from diplomats in the course of negotiations, so it wasn't done.