I couldn't quite get it working, maybe BlueW can answer better...
For First question
Yep, you need to edit the /etc/nodogsplash/htdocs/splash.html file.
You can find an example here:
http://nodogsplash.readthedocs.io/en/latest/authentication.html
You put in whatever authentication you want in that file.
Here is a nice example
Add something like this to /etc/config/nodogsplash
PasswordAuthentication yes
UsernameAuthentication yes
Password nodog
Username guest
PasswordAttempts 10
It didn’t quite work for me, it allows any username and password
To view your splash page for testing use this link:
(Change IP to your router if needed)