Electronic – PSU Current on negative line

To my understanding, the voltage supplies the potential

Voltage is the difference in electric potential between two points. It is analogous to a difference in pressure between two points of a hydraulic circuit.

and free electrons are taken from ground?

Free electrons are everywhere in metal. All matter contains electrons, and in metals, some of those electrons can flow freely. They won't flow until you apply a voltage, though. Applying a voltage causes them to flow, which we call an electric current.

http://amasci.com/miscon/eleca.html#batt

"Ground" is just a label for a point in a circuit. Any point can be considered ground. It's not special, it's just a common reference point for measuring.

Another question, so mains line provides the potential of 120/240VAC (ignoring AC) and the current does not actually "come" from it, only from ground?

Current doesn't "come from" anywhere. Current is a flow of charge, which already exists everywhere.

In electronics, electrons aren't usually important. What's important is flows of charge, waves in charge, etc.

Go read everything on this page: http://amasci.com/miscon/elect.html

Related: Why does a resistor need to be on the anode of an LED?

Also related: Why is the charge naming convention wrong?

"Output current" should be read as "current into the output pin". As such, negative values indicate that the pin is sourcing current, and positive values mean that it is sinking current.

The floating issue can be remedied by using a device with bus hold such as the SN74LVCH245A. The bus hold feature holds the input pin at the previous detected logic level, and since the inputs and outputs are connected internally in the '245(A), this will hold the output as well regardless of the state of nOE and DIR.