Can a 100' string of led lights be powered from each end hooked to one power supply
120V or any mains voltage - no no no no no no no!  

5 or 12 volt DC, yes, and at 100' you will need to feed it from many intermediate points as well.

24 volt DC, along 100 feet you will not only need to feed both ends but several intermediate points too.  Just not as many as 12V.