I'm so sorry you lost the little one, you fought so hard for and with him

I think in this short time span the food can't be the cause.
It is really difficult to raise those little ones.
Perhaps the chick got internal damage when falling from the lamp to the ground.
We will never know.
You did your best and you helped him and gave him a chance - it is not your fault the little one couldn't make it.
Even very experienced people often loose their little proteges.
Thank you for trying everything for this and the other little chick
Watery poop can be a sign of stress. And first poop in the morning can be a bit more watery than other poop.