the 10m and 20m have been on the parts list for a while but not available the last I checked. I haven't heard of a 7m version of the cable. But the 10/20 look like they will have their own firmware on the cable so possibly another firmware to update on a system. Hopefully we get them soon... the Atto bridges are only used for MetroCluster so can't use them to extend length over 5m.
Last shelf needs to be connected for dual controller, yes. Single controller does not need it.
And having 10 x 20m cables does not mean having 200m distance. I have seen enough configurations where long cables were ordered "just in case" because at the time of putting it together nobody could say where equipment would be actually located. Then you have to use whatever is available on site. (I remember couple of dozens 50m FC cables between adjacent cabinets. Just to find place to stash them away was the challenge )
And at the end the real question is, would NetApp support it. I have seen mix of FC and SATA shelves in one loop, shelves connected to non-shelf port on FAS270, copper controller-to-shelf connection. It worked