If all relief pitchers are below their auto rest lvl, will sparky use the least fatigued?

No, sparky will use them in their designate role and as it sees fit to use them. Their level of fatigue becomes irrelevant if they are all fatigued. My recommendation is set your two worst pitchers to 0 autorest start 1 with no PC and a pull 1 an set the other in the pen with no PC and a pull of 1. After a game or three your fatigue issues will be straightened out.

^ I agree with the first point, sparky will choose a pitcher by designated role first. not sure about the second point, fatigue may be a factor when choosing among pitchers of the same role.