key rotation is complicated, if one member doesn’t receive the message with the new key they will not be able to read the group anymore, as I already mentioned in the other thread, it is better to keep things simple:
Would key rotation be easier in the “blind” groups? As in the owner could help co-ordinate the key rotation to provide many-to-many comms, not just 1-to-many.
it is the same problem, you have no warranty all your subscribers received the new key, so you can’t rotate, you could implement some confirmation mechanism in which you pre-advertize the upcoming key but keep using the old one until all subscribers confirm they received the new key, but just one inactive member would delay the rotation potentially forever, plus such auto-reply/confirm mechanism for subscribers have privacy considerations
it is a blind group == channel, key rotation is not needed, it is probably public channel, if used privately for people you trust then it doesn’t matter
It will be easier without this confirmation part.
Admin adds a second key, and in a couple of days/weeks drops the first one.
that would cause “unable to decrypt” experiences, which is not acceptable
Am I correct in understanding that newly added group members won’t see previously published messages in the group? Is this the same for channels?
yes.
yes.
for groups, this is can be regarded as a privacy feature. however, reason is also that it is just not done and not easy 
still, one can resend one own’s messages after a member is added.