1a. Yes it’s mandatory to have at least one stream node on 8.x versions.
2a. To start with, the OOTB Queue processors will fail and elastic search won’t function.
3a. Same as above. They won’t function in Dev. BackgroundProcessing Or Universal is required for agents to run.
In conjunction with above responses, note that by NOT setting any nodeType like Stream or Universal, Pega automatically assigns 4 node types Webuser, Backgroundprocessing, Stream,Search to avoid breaking functionalities.