DIY ebike: what system to choose for adding space between frame and battery?
I think a problem with option 3 is that the screw’s tightening torque is “pulling” the rivnut out of the frame (in addition to any force acting on the battery). With option 2 the frame only receives the actual forces acting on the battery.
Sideways stability should come from your mounts being several centimeters apart, not from using wide washers/spacers.
In all of this I’d make sure to use threadlocker and sufficient tightening torque (like &gt;4Nm) to make sure nothing comes loose and starts weakening over time.
Oh, and one idea: Instead of rivnuts, why not drill through the whole frame and use very long bolts and a bracing plate (or wide washers) on the top and bottom? Or even just clamp plates around the frame without any drilling and mount the battery to the plates.