Since scales inherently involve putting weight on them it should be possible to use that weight to push down a plunger generating enough electricity to run for the weigh-in.
If there's a plunger resisting the scale going down to harvest energy, that would prevent all the force from being on the load cell, giving you incorrect readings.
Not if the plunger rests on the load cell. Then plunger just captures the work done as it is compressed, it doesn't reduce the force that passes through it (once at equilibrium.)
A spring would then reset it for the next time.