On the Seagard question. Are there any historical examples of ports which existed to portage goods overland to avoid lengthy sea voyages comparable to your description of Seagard?

Yes. There is the Diolkos in Ancient Greece which connected the Gulf of Corinth and the Saronic Gulf, the Vikings in Russia established portage routes connecting the Volga, the Western Divina, the Dnieper, and the Don, and then of course there’s the portage routes in French North America that fur traders and trappers used to get their products to market, given the frequency of rapids and waterfalls that made straightforward riverrine transport difficult.