Is indeed more Alfresco-related. I will try to answer, nevertheless:
A task can only be edited by the person that is assigned OR by the process initiator. This has nothing to do with permissions on the models, more of a service/ui constraint. The content-model uses for tasks describes the task, however, the task is NOT an Alfresco-cm:content but just a task in activity, having no Alfresco ACL's capabilities.
A solution should be to:
- Create a multi-instance (as parallel-review) that has a task for EACH reviewer assigned to him/her explicit ally.
- Add a service-task before the tasks, setting READ_CONTENT permission to all selected assignees
- Add a end-task-listener to the review-task, that revokes (if needed) the READ_CONTENT permissions from the document again
Or am I getting your use case wrong?