"with two player, the machine stops after the third total ball (i.e. first players 2nd ball)". Does it do a normal game over with a match number lighting, or not lighting? The latter would likely be the slam tilt circuit or something else forcing it to game over vs. normal end of a game.

I fixed up a Magnatron and kept it a while, never knew that ring was supposed to go in front of the balls like that. It is a very weird-looking arrangement for a captive ball, probably why I assumed the captive ball should be 'open' since that's how nearly all of them are on other games.

The Last Ball relay is activated when the wipers on the player unit complete the circuit at the last ball (3 or 5 as per adj.) so make sure that is correctly aligned i.e. you are getting the correct ball #s displaying for both players, especially since you noticed the disc had shifted off the rivets a little.

The special usually will stay on for the whole ball on a multiplayer like that but might alternate between two targets or holes, and depend on lib/cons settings or 3 vs 5 ball. Plus you should have conservative/liberal settings available on the upper rollover posts (and elsewhere like around the targets and side drains) to make ABC more difficult to achieve.

I just noticed the instruction card on a Duotron on IPDB has rules about drop targets. The game doesn't HAVE any drop targets! It's like they used a card from another game and stuck 'Duotron' on top of it but I can't figure out what game it IS from.
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I meant the physical positioning of the playfield posts themselves, some will have an extra screw hole so you can make certain things harder to hit. Can probably adjust the upper rollovers so the center one is higher and thus ABC is more difficult, or lower the ball times by adjusting the drain posts to be harder but the manual will have a diagram of exactly where those adjustments are. Looking at a pic of a stripped playfield it looks like you can adjust the upper posts on the slingshots and the upper center rollover on this game but you may already have them in the conservative position.

Also there are lib/con adjustment holes for the posts in between the ABC targets as well, looks like it anyway.

See arrows, if no one ever changed them from the factory on your game you will only see dimple marks, but those are the alternate screw locations.

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The slingshot posts have the two screw holes, the rails are fixed. Opening the drain more makes the game more difficult. You can also experiment with going to 3 ball and making the playfield posts super liberal, or stay with 5 ball and make it more conservative, or somewhere in between. Changing a post just a little especially around the drains can make a surprising difference in how a game plays. I rarely set a game to 3 ball but sometimes it fits.

Might be able to slightly tweak the wireforms next to the side kicker arms, and/or the kicker arms themselves, to help straighten out the kick, or kick it more in the direction of the siderails instead of the posts.