score:4
We ended up dialing in a video call from another device and walking her through the entire process, which took literally three (3) full hours, including no less than 30 minutes to enter an email address correctly. (I was anticipating trouble with spelling and the "@" sign, but I did not expect that finding the Western "." character on a Japanese keyboard would be so hard!). But the marathon paid off, the ETA was granted immediately.
Slightly more usefully for future readers in the same pickle, our Plan B was to hire a benriya (δΎΏε©ε±, lit. "convenient person") to do the job. Major chains include https://otasuke365.com/, which has outlets all around greater Tokyo and rates from 3,300 yen/hour.