I have to test that, I was of the impression that Spirit Guardians end as soon as you cast Sanctuary. I had this twice in fights lately, but maybe there were other reasons for the end of the concentration. An untouchable Spirit Guardians caster would indeed be nice (or: totally op). As Shadowheart is in my group permanently, she should perhaps always concentrate on Spirit Guardians instead on Bless or Call Lightning.
My Warlock build is a crit build, therefore Fighter 4. Action Surge is just a nice addition. I'm always considering wether Fighter is worth the lost feat which I could get with Warlock 4/ Sorcerer 8.
