@louis Doesn't seem to offer much at this point; also, ethics code is quite weak and what if you want a stronger one than another member is willing to enforce?
Yes B can either drop out or they can silence the server but remain federated, up to them.
There is no penalty for joining and leaving though, so worst case scenario you are back where you started, no harm done.
Keep in mind the UFoI is not meant to include the whole fediverse. We picked rules that would allow the vast majority of instances join, those that want to be part of a larger community. Other instances are welcome not to join, there is no "penalty" for not joining.
Long story short, no harm in joining up until you want to defederate, then just leave if it ever comes to that.