As there hasn't been a response from Topaz, here are some answers I have summarized from other threads.
Your question: "By this I mean: what is the benefit for users who already own the existing plug-ins and use them from within Photoshop (which gives them all the masking and adjustment capabilities that they could wish for)?"
In this case you would be using TS as a plugin from PS for the unique Adjustment combinations it offers, as you utilize other plugins.
As was stated, "Topaz Studio will be the new one-stop program for all of Topaz Lab's technology. Through this platform we will be able to very rapidly implement the latest in photography manipulation in the form of affordable adjustments without having to create a brand new products/plugin for each bit of tech. Topaz Studio is supposed to simultaneously alleviate our inability to just keep making new products and deliver a valuable image editor to our customers."
A further explanation of the "future" can be found here:
And then Albert Yang posted this thread:
Your question: "How does one stack effects one on top of another? I thought that if I duplicated the processed image in the workspace and applied another effect to the duplicate, it would apply a new effect to the processed duplicate. But the second effect is applied to the original. The bahavior is different using plugins. If I apply an effect in TS and then call one of the Topaz plugins (like Simplify), the plugin applies its treatment to the TS treatment. This is very confusing."
In a similar way to using Photoshop, where you would create a "stamped" layer after applying adjustments and before calling external plugins, in TS you use the 'Apply' button to create a new image which you would then pass to the external plug-in. Duplicating the image will just create a duplicate of the base image you are working on without applying the existing Adjustments.
Your statement: "I couldn't find a way to reset a mask (to all white) or to correct a brushed-in mask. It took me a while to discover that the * slider "Mask Density" changes my brush from black to white so I can use it to paint in or paint out the mask. As a user familiar with other masking software, I didn't find the term "Mask Density" to be very intuitive. (EDIT: in the meantime I found the reset button to reset the mask).
The Edge Aware slider seems to have no effect using the masking brush. Edges are ignored whether it is turned on or off. I haven't discovered a way to apply a mask to a precise selection."
On resetting the mask it is a little convoluted as you need to use the invert button, if necessary, and then reset as you noted.
As you pointed out the use of the mask density slider as the greyscale indicator from white to black is confusing, the simple use of a greyscale slider would be sufficient.
The edge aware button does seem to work ... unfortunately though the red overlay doesn't reflect this.