Yes, everyone affected should be allowed to testify at the Brexit hearing. Their views should be heard and considered, especially since it appears they were not heard during the original vote. If there is going to be a review based on the fact that not all sides were considered carefully, then more effort should be made to be thorough this time.
They have a rights to decide whether or not they want to leave the EU as well. The United Kindom is made up of multiple, independant countries and these countries deserve a say in the decisions that effect them. Furthermore, they should be allowed to decide if they want to leave the UK.
Yes, all countries affected by Brexit should be allowed to testify because the decision affects the entire world. No one should be happy with the decision, and in many ways, Britain should hold a new vote. This should not be the path Britain chooses, to break away from the EU.
I do not believe each country should testify. It is up to Britain whether or not they choose to stay in the EU. They must make the decision that is in the best interest for their country. Britain should not be forced into staying into the EU by allowing other countries testify against Britain.