We could definitely make it with an eVent base as well, shoot us an email and I can give you a quote.
The seam sealing is done with seam tape.
We're starting to design one with a side zip entrance as well. I'm trying to come up with something similar to our normal side zip bivy, using a waterproof zipper. Hopefully have it made up and ready as an option in a week or so.
The bug netting is a removable panel that is under the fabric hood. When the bivy is held up using the pull loop, it actually gives quite a bit of space underneath.