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I see that Nick has already provided a good answer to R5, so I'll only address this:

<blockquote>Furthermore, an amplifier symbol called "gm" is labeled as transconductor. What does it do?</blockquote>

What you show is a rough diagram only intended to give enough insight into the internal workings to use the power supply effectively.  A lot of details are left out.

You say:

<blockquote>ability to chain channels in parallel to increase current output</blockquote>

You can't just connect ordinary constant voltage power supplies together and expect them to play nice and share current.  This is a capability that must be designed in, and requires something different from typical topologies.

My guess is that the output stages are indeed current sources.  The feedback loop controls the current source to maintain the desired output voltage.  When multiple supplies are connected in parallel, their feedback signals are also effectively shorted together.  That way, each supply will produce roughly the same output current as the others.  The overall feedback still adjusts the control inputs to these current sources to cause the desired output voltage.  If you assume that the output voltage is held constant and that each supply puts out about the same current, then all the supplies share the load current nicely.