Changing crankset - axle length
The cranks are fixed at the ends of spindle, so the spindle length affects chainline. The final chainline is determined by axle length and crank design. In your case, your existing spindle is 5.5mm shorter than the recommended one, so the chainline would be 2.75mm inward from the spec and match the chainline of the previous crank.

Cranks and chainrings are made of aluminum, which does not rust by itself. You should be able to remove rust from chain by wiping.