Questions regarding removing a secondary node from an availability group
Yes.  Primary benefit is that you can keep the listener online.  Downside is that there is still a dependency on the cluster service to keep the listener online, but that shouldn't be a significant risk since there is only one node as it will always own quorum, assuming you remove all quorum votes other than the remaining node's vote.
The listener is a component of the AG, so if you delete the AG, it will delete the listener.  Options are to 1) just remove the DBs from the AG, 2) you can delete the AG and then create a new cluster role that just has the network name and IP address (which is really all that a "listener" is), or 3) delete the AG and add a DNS record for the host name that clients are currently using.  You could also do as you suggest to modify client connections, but I don't know how painful that would be for you.
If you leave it as a one-node cluster, you can simply join a node to it in the future if needed.  If you remove the cluster completely, you simply have to create a new cluster and join the two (or however many) nodes to it.
If you pointed the SSRS instances to the listener name, then you don't need to do anything if you keep the listener or add a DNS record for the listener name.


With your comments about possibly using an AG again in the future, I personally would not delete the AG and cluster.  I would remove the databases from the AG, remove the secondary node from the cluster, and remove all quorum votes except for the one node in the cluster.  Less work now, less work later.

You haven't mentioned anything about how backups are occurring so there's no way for anyone to know whether your backups will continue as expected.

Also, you'll first have to remove the databases from the AG prior to deleting the AG.