I was able to say to Tony last week: help, I have cocked this up quite thoroughly and now I can't even boot the thing, and I am so upset about being stupid I am making myself even more stupid. Can you please take this from me, and hopefully as you aren't all emotionally tangled up about it, you'll find it straightforward. Please make it boot again, and reinstall Ubuntu, and once I have Dropbox installed I can take it from there.
And he did, by the next day. (I was sufficiently busy and stressed out I avoided checking it until the weekend, but it works! Huzzah. I have lost a small amount of data because my backups are only weekly, but nothing disastrous. Thank goodness.)
 Truly, I didn't think this through kind of stupid, yes