Too many things going on here.
Firstly, “routed the belt correctly”??
Secondly, “rattling noise”??
Thirdly, “black coolant”, “down 2 qts”??
Fourthly, if there is oil in the coolant, what’s in the oil?
Fifth, “battery low”.
Lastly, where are the codes?
I’ll say it; Perhaps you took on more than...