In regards to your second question I personally try at the start of an encounter(where shield lob doesn't permit/after shield lobbing) to spirits within > circle of scorn > combo. After that I'll never use them together. My reasoning is the animation on spirits within feels very slow however the circle of scorn animation feels like it finishes in a much shorter time than the spirits within animation. So shield lob > spirits within > circle of scorn > combo. However during the actual encounter I'll seperate the instant casts between gcds as it feels much more sluggish using 2 instants between gcd. This is just how I personally use my skills though, there'll be other paladins who will prefer to keep 1gcd apart at all times, and others who will use both inbetween cooldowns. For threat it's not a majorly huge issue as if you're losing threat because of this you're probably doing something else entirely wrong, and the damage doesn't really factor in since you're a tank and the damage you're losing/gaining is negligible.