So, I actually managed to figure it out 🙂 Was easier than I thought.

For future reference, should anyone need it:

• Place the shaft back (I secured it with some light tape from the bottom, to prevent it from falling back in)
• Insert the black ring and center it as much as possible with tweezers. 
• Carefully insert the bearing balls. Take your time to place them, it's a fiddly job.
• Insert the white ring, with the slotted part down. This lines up with the slots in the black ring. I found it easiest to gently push down the lift mechanism, so I could see if they lined up
• Re-install black retainer clip

After this my initial issue was still there, which was that the player was stuck in the same groove. Arm was balanced correctly. Fearing a broken element, since I bought it from eBay, I checked one last thing.

The word abused in my original post was not for nothing...

The return mechanism arm was bent upward, so the tone-arm actually had a very hard time moving. I carefully bent it back (after removing it of course),et voila... It tracked perfectly at 1.2 grams! 🤩

All in all, it was it pretty good shape, considering it's 35 years of experience. Even the "felt" on the idler wheel adjustment arm was still in place and in good condition. The only thing I will replace is the wheel itself, since the rubber is rock solid.

I'd say a very good buy for 30 euro's 😕