I believe the metal balls are lead or another heavy metal, added simply for weight to keep the base down. They are certainly not mercury. As long as you got the majority out they shouldn't cause any problems in the tank.
As for the puffer the swallowed glass will certainly have an effect, the metal not so much. If the puffer continues not to eat it's likely dieing and you should consider doing the humane thing at that point.
Sorry for your luck.
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