You need to look at the verb and see who/what does the action which you then find the subject so you want to find the simple subject
Sustainability is important for a very simple reason: we cannot maintain our quality of life as human beings, the diversity of life on earth or earths ecosystems unless we embrace it. In order to have healthy communities, we need clean air, natural resources, and a nontoxic environment.
I believe that the two sentences (apart from one with the brackets) are correct. So, those would be:
1. <span>Daniel, that guy with the hair, is sitting at the end of the hall.
2. </span><span>Daniel—that guy with the hair—is sitting at the end of the hall.
Using commas and dashes to separate the appositive phrase from the rest of the sentence is grammatically acceptable, whereas using brackets is not.</span>
