Disconnecting/reconnecting the domain worked for me. It still took about 10 minutes and you have to change the verify record.
This is after it was in the "processing" state all night. I don't think whatever is supposed to retry the LetsEncrypt process is working.
A couple of things that may help:
1. set the TTL as low as possible on the domain records you want to change a few hours before you want to make the switch.
2. make the DNS change within a few minutes of "connecting" the domain on squarespace.
In any case Squarespace, 72 hours is not an acceptable window in many cases.