I've been playing around with the Google build on Hwb this week and it seems pretty well put together. I've put the link to my full article on it below, but the TL:DR takeaway is that there's no obvious reason why a school already confidently using G Suite would switch over (lack of Gmail and, weirdly, Jamboard would mean losing features) it's great for the majority of schools (esp. primaries) who have not the experience or expertise in setting such things up themselves.
First Look at Google on Hwb - MrAaron
As for Hwb DP forms, the Head one is needed by all schools. The pupils one only needed if you're going to use their services, they are NOT needed for the compulsory online tests etc.
