Spark and timing occurs as a function of a fixed magneto position relative to crank position, not mixture setting. The spark fires at the same point BTDC each time. Preignition isn't an issue, though detonation can be. The spark doesn't advance, and there isn't use of a variable timing or advance mechanism on most aircraft engines.
Can you elaborate?
With respect to fuel efficiency at lower RPM's...yes, lowering RPM means lower fuel consumption; the engine isn't turning as fast. However, if one is lowering RPM's by reducing throttle and thus reducing manifold pressure (which is the subject of this thread)...one also isn't getting the engine power...one isn't going to go as fast, climb as fast, and therefore a significant increase in specific fuel consumption isn't necessarily realized...nor is the rate of fuel consumption, or the efficiency, linear.
If one reduces RPM by use of the propeller control, one can't simply point to the lower RPM as the cause of any perceived or actual increase in fuel efficiency, and relative to manifold pressure the point is rather muted. If one is reducing RPM using the propeller control, one is increasing blade angle while maintaining or increasing manifold pressure; one is increasing the effect of thrust imparted by the propeller, and thus increasing the efficiency of the operation, to a point. This really has nothing to do with increases in altitude and decreases in manifold pressure, however, and one can't attribute the increase in efficiency, where this does indeed occur, simply to the decrease in RPM...it's attributable to the change in blade angle, and the change in RPM is a nearly insignificant factor in the matter. The change in blade efficiency and effectivenes, however, is a big player in the matter, and accounts for the change in specific fuel consumption relative to distance traveled.