I was thinking about that too. They are set-up like the diagram now. STP is blocking where it should be with the redundancy between the MX and the 355's, but that is good point. The 355's are stacked. I can advise them to remove two links like you mention to reduce potential STP issues from occurring, but so far so good.
I was concerned about using the public IPs on the WAN switches. Any concern there beyond locking down the local status pages on those devices? Not a huge fan of public IP's on the WAN switches for management personally.