Do those people that opposed him be given a chance to voice their complaints?
This is a basic misunderstanding of what is the Covernant of Bahá’u’lláh.
There are no complaints that can be lodged.
With the Will and Testament of Abdul'baha, he was given full Authority to appoint who he saw had the qualities for such an appointment, the provisions are given to be fully embraced and followed, there is no disputing them.
The opening of the will was done with full transparency.
The Qualities of Abdul'baha have never before been seen in another human, apart from the Messengers, He was given as a perfect example for humanity. Those qualities were well reflected in Shoghi Effendi and very few people can reach these levels of sacrifice of self, for the good of the whole.
The only way we get to know of Abdul'baha and Shoghi Effendi is to read from the people that met them and served for them.
Regards Tony