It means the key for the mycroft ppa has expired. You will also get this if you run apt-get update
It isn’t something you can fix. Mycroft needs to update the keys. I assume this is effecting everyone.
I don’t know. I haven’t done that, but reintalling shouldn’t help the keys. So unless you have another reson for doing that, I wouldn’t bother.
I eventually found the 0.9 image, but on this page there are two links in the Picroft section… one which takes you to to the enclosure-picroft Git repo, and the other is which leads to the bad image I downloaded. Unfortunately I can’t include the links themselves because of new user restrictions.