Assuming the city state has adopted the correct religion, is it possible to have a resting point that's permanently friendly?
Yes. Most things in Civ 5 stack as you'd expect them to. For instance, if you also pledged to protect the city state, your resting point would be +45.
The exception to this tends to be multiple bonuses which are the same e.g. the bonuses from two great generals wouldn't stack, the penalty from flanking can only occur once.