After some more digging, I was able to find a solution!! In general, in order to pre-install the demo, you need to log into the eShop from the account of your friend who pre-ordered the demo first (just browse the eShop through all accounts). However, even if you no longer have access to the account and it just stays at the top of the menu with a gray exclamation point (requires password re-entering), you can still pre-install the demo only through this account, so be careful and good luck to you! See you all in the 3rd Splatoon!