I would actually prefer the use of an IGP to handle the failover between site A and C, but there may be limitations with doing that. You could combine that with two active GRE or VTI tunnels and it would work great for the A to C connection.
Your question is along the traditional IPSec side. You may want to look at the following URL which discusses the new stateful failover features in the later 12.4T.
If money were no object, I would do A to B with two ASA's in failover and reverse route injection and the A to C with VTI and an IGP protocol like (EIGRP or OSPF). The ASA has a great stateful failover.
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We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...