I answered my own questions which I will describe below for anyone who may be interested.
I changed the default admin password, just to be safe, and set up a new user and password.
I found a post on reddit that described the following steps for accessing calibre-web which worked from my Kobo Clara HD's beta web browser:
- In the Kobo browser, enter URL: http://192.168.0.100:8083/login (or whatever your IP happens to be)
- Enter user and password. I used my new user's credentials. Click Login.
- This should produce an OMV page.
- I then re-entered 192.168.100:8083 (without the /login part) and this brought me to the calibre-web portal
The Kobo browser is beta (and has been for years!) so the experience is terrible but you should be able to navigate around and download books of your choosing. They will save to your device and open as expected!