If I am in a situation by which my PUB is down, are users still able to make changes to CallForwarding on their lines? My first inclination is that it is not possible as the SUB is a read-only database but I wonder if anyone knows definitively.
You are most correct here :) If the Pub is down or Replication between the Pub and Subs is broken no changes can be made to CFWD ALL. Here is a doc that speaks to this question;
This is a list of possible symptoms if the subscriber stops replicating from the publisher:
Changes that are made on the publisher are not reflected on phones that are registered with the subscriber.
Outbound calls fail on phones registered with the subscriber. As soon as you dial 9 you hear a re-order tone.
Call Forward All (CFwdALL) does not work.
IP phone displays Error Database.
From this doc;
Hope this helps!