Antaran,

 Like your model so far. Here are my comments.

 

Quote - 3. Do I need to UV map this and how do I do this right?

Right now if I go into the UV map view, the thing looks scary. And I have no idea how to make any sense of it.

 You don't have to UV Map it as it looks okay now, maybe add another "Basic Layer" on top of the handle in the Texture Room with an opacity of 50% or something . Layered shaders can add depth to your textures.

  Here is a quick way to UV Map something in the VM room with multiple meshes.

1.Shift + Select one "Edge" from each of you separate polymeshes in the vm room.

2.Dynamically extrude these edges , doesn't matter how much.

3.Ctrl+shift+w  and weld all the selected points together. You should be left with a single vertex.

4.Delete this Vertex.

 What we did here is connect all of our polymeshes into 1 polymesh but they are still separate polymeshes for selection purposes.

5.Now double click the entire polymesh ,click the UV icon in the VM room and choose Operations/Box/6 sided mapping.

Your whole object should be mapped although you could scale the UV's if you want.

 

4. What about scale? Right now this thing is huge. Where do I scale (assembly room? modeling room?) it and how do I do it correctly for best usability?

 It is probably best to scale it in the Modeller but I generally scale things in the assembly room as it is quicker. If you like to have your models scale consistent with other models in the scene then use the correct scale in the modeler. 

5. This is a single vertex object with multiple sub-objects and shading domains. Is this the right way to do it or should I be separating all the internal objects into assembly-room objects and grouping them in the assembly room?

This is how I model most things in the Vertex Modeller.

I am also using Carrara's smoothing. Is that a good thing to do or a bad practice? The model does look a bit different when smoothing is disabled. For example, the glass does not match the frame when smoothing is changed.

You can smooth both the Glass and the frame which sometimes makes things fit together better.

I'd like for someone here who is better experienced at modeling to take a look at the model itself and tell me what I can improve about it. Hence the question of how and where to upload it.