I agree with your understanding of the schematic. A playfield switch will keep the point relay active even if the Score Reel EOS switch is open.

I suspect that the longer solenoid is the cause of the trouble. When it first fires the bell solenoid pulls the plunger up through the solenoid and into the side of the bell. Normally the Score Reel EOS switch acts just as quickly so as soon as the plunger hits the bell (or very soon after) the power to the bell solenoid is cut and the plunger falls back through the solenoid to its rest position.

But if a playfield switch is held closed and the point relay stays active for a longer period the plunger doesn't fall back to its rest position because the solenoid too is still active. Instead what happens is that the magnetic field in the solenoid wants to hold the plunger up longer which brings gravity into play.

When it's first launched at the bell the plunger gets sucked into the solenoid and actually overshoots the strongest part of the magnetic field at the center of the solenoid on its way to hitting the bell. Once it's above the solenoid and in a weaker magnetic field gravity starts to do its thing and the plunger falls back through the solenoid. Eventually it reaches the strongest part of the magnetic field again where the magnetic field is stronger than gravity, but momentum makes it overshoot again through the lower side of the plunger. The plunger will actually oscillate back and forth through the solenoid several times until it settles at the point where the upward force of the magnetic field matches the downward force of gravity. You can see the oscillation if you look closely at the video at https://www.funwithpinball.com/exhibits/small-boards#Solenoid (Note that the solenoid in the video is weaker than a bell solenoid, but the physics is the same.)

Since it is an AC coil the magnetic field that acts on the plunger is constantly changing which further complicates things, but the gist is that the plunger takes a little time to settle down.

If your new solenoid is closer to the bell it may be that the bell is now close enough that the plunger is hitting the bell on subsequent passes through the solenoid. The initial kick will travel the most and each subsequent oscillation will travel less until the plunger finally stops. Raising the bell might put it far enough away that the plunger can only hit the bell on the initial kick.

/Mark

A diode would only get you half wave rectified AC, or essentially half of the AC signal. So the solenoid would be on half the time and off half the time. The net effect might be that the plunger is fired with less force which might end up reducing the buzz. You probably wouldn't damage anything if you tried but I don't remember hearing of that being a solution in any similar situation.

The alternating current may be a contributing factor in the plunger oscillation but I'm pretty sure that it's mechanical inertia and the overshooting past the center of the solenoid that is the main factor. Plungers in other devices only stop because there is a mechanical stop built into the device (pop bumpers, flippers, sling shots, etc.) The bell plunger has no such stop other than the bell, but it doesn't stop the plunger in the sweet spot of the solenoid like the others.