For the first part there is an elegant solution: force ongoing connections to initiate from the static ip of a controller. This behavior is ensured when preferred_lft 0 is set on an ip address of an interface (as described here: http://www.davidc.net/networking/ipv6-source-address-selection-linux).
This would allow us to assign a floating IP and a static ip from the same subnet to the management network but at the same time make sure that all outgoing connections to ip's on other hosts in this subnet are initiated from the static ip.

In the example below we have the active controller with 3 ip addresses on the management interface, one static and two floating. The floating IPs move from controller to controller on swact:
1. static: fd01::2/64
2. floating: fd01::1/64
3. floating:  fd01::4/64

 Connections w/o the option:
====================
[And give example here]

We can see that there are connections opened from the floating IPs.

 Connection with the option:
====================
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name    
tcp6       0      0 :::6800                 :::*                    LISTEN      2587586/ceph-osd    
tcp6       0      0 :::6801                 :::*                    LISTEN      2587586/ceph-osd    
tcp6       0      0 :::6802                 :::*                    LISTEN      2587586/ceph-osd    
tcp6       0      0 :::6803                 :::*                    LISTEN      2587586/ceph-osd    
tcp6       0      0 fd01::2:6789            :::*                    LISTEN      2587301/ceph-mon    
tcp6       0      0 fd01::2:40924           fd01::3:6789            ESTABLISHED 2587586/ceph-osd    
tcp6       0      0 fd01::2:40888           fd01::3:6789            ESTABLISHED 2587301/ceph-mon    
tcp6       0      0 fd01::2:45520           fd01::3:6800            ESTABLISHED 2587586/ceph-osd    
tcp6       0      0 fd01::2:43872           fd01::3:6804            ESTABLISHED 2587586/ceph-osd    
tcp6       0      0 fd01::2:6801            fd01::3:47520           ESTABLISHED 2587586/ceph-osd    
tcp6       0      0 fd01::2:6802            fd01::3:53458           ESTABLISHED 2587586/ceph-osd    
tcp6       0      0 fd01::2:6803            fd01::3:48860           ESTABLISHED 2587586/ceph-osd    
tcp6       0      0 fd01::2:48158           fd01::15f6:1bc9:22:6789 ESTABLISHED 2587301/ceph-mon    
tcp6       0      0 fd01::2:58020           fd01::3:6803            ESTABLISHED 2587586/ceph-osd    
tcp6       0      0 fd01::2:34342           fd01::3:6800            ESTABLISHED 81962/ceph-mgr      
tcp6       0      0 fd01::2:45484           fd01::3:6800            ESTABLISHED 2587301/ceph-mon    
tcp6       0      0 fd01::2:48000           fd01::15f6:1bc9:22:6789 ESTABLISHED 81962/ceph-mgr

How to configure the option by hand
===================


Result:
6: vlan10@enp0s8: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc htb state UP group default qlen 1000
    link/ether 08:00:27:66:a6:e8 brd ff:ff:ff:ff:ff:ff
    inet6 fd01::1/64 scope global deprecated
       valid_lft forever preferred_lft 0sec
    inet6 fd01::4/64 scope global deprecated
       valid_lft forever preferred_lft 0sec
    inet6 fd01::2/64 scope global
       valid_lft forever preferred_lft forever
    inet6 fe80::a00:27ff:fe66:a6e8/64 scope link
       valid_lft forever preferred_lft forever