Can Ink Treader Nephilim's Electrolyze copies target players?
No, you cannot, because the card specifically states that each copy targets "one of those creatures." So the target for the copy is specified in the rules of the card itself, there is no option to choose a different target.

Arc Lightning is the same, but Liquid Fire works a little differently. With liquid fire, there is only one target, the creature. How much damage is done to the creature vs the creature's controller is chosen as you cast the original Liquid Fire, and that same chosen number will be applied to the copies. (By rule Rule 706.10). So if you chose "x = 0", thus dealing 0 damage to  Ink-Treader Nephilim, and 5 to  Ink-Treader Nephilim's controller, then 0 damage will be dealt to each creature, and 5 to each creature's controller. (So players who control multiple creatures will take 5 damage for each creature they control, not just 5 damage total).