Maybe this insight will help

"I had reached a point where I needed to be able to refine some features at a level that seemed unavailable with the DAZ toolset.  Most of all, I needed more control over morphing the nose and mouth."

Thoughts:  Not sure what types of lips and noses you are trying to create but there are additional morphs available for V4 based on what you are trying to create.

"so far I have found the fitting process to be extremely frustrating, as the conforming feature isn't really working since I've made a variety of custom tweaks to various parts of the body shape."  

Thoughts: Depending on what version of Poser you are using you'll either need to use the morph tool or fitting tool (fitting morph tool is only in the newest release of Poser).  Depending on how extreme your morph is, you may need to add the new morphs to the clothing.

Are the steps I've taken so far for my character design a good workflow?

 Depending on what type of character you are designing it sounds about right meaning that there is no wrong or right way to go about designing characters.  Some people start with a pencil and pen, while other use clay.  There's only working and shaping the character until you get what you need.

Should I have built the character from a zeroed V4.2 base instead of starting from the custom themed character?

I'm not sure what you mean here. Are you building your character based off another character that you purchased?  If so there may be limitations to the character that you purchased and not to the v4.2 character.  For me I build my characters from v4.2 without the character morphs used.  The only reason I buy characters is for the skin.  The only reason I start from v4.2 is because it allows me to have a fresh start in terms of creating unique characters.
Should I just bring the character (in it's current state in DAZ) into something like ZBrush to finalize the finer details?  I know exactly how it should look -- I suspect there's probably some magical combination of dials in Poser, but lordy it sure seems like a bit of a crapshoot sometimes.

Sometimes it is a crapshoot but the more you work with the dials the more familiar you'll become with what changes the dials will make.  I actually mapped many of my dial settings so that I can have a visual of what that dial change looks like before I even make it.

Is this what I should expect when using 3rd party content with such a customized character or are there steps I'm missing or doing incorrectly that would make this process less tedious/frustrating?

It doesn't sound as if you are doing anything incorrectly.  Most of it seems to be more of a process of becoming more familiar with the changes that the dials make. 

Is there some glaring thing that I'm doing wrong, forgetting, etc?!

I don't think it's so much about your workflow as it is your familiarity of the morphs that you are using.  The only other options that I know is to sculpt the character so you don't need to mess with the dial.

I'm curious.  what type of lips are you trying to create?