I'm studying both simplified and traditional characters.

How does the word count progress graph work in this case? If I can write a word in both simplified and traditional forms, does that count as two?

Yes, it counts as two items learned for the writing, but not for the other parts--except for when the reading, tone, or definition are different between simplified and traditional. This only applies to a few variant characters.

Thanks Nick!

So the one thing I'm curious about is how many unique words I've learned how to write, without regard to whether in simplified, traditional or both.

Am guessing I can't just divide by 2, since many words are the same in both forms.

Your word definition or tone learned numbers should be pretty close, there.

Good point...

I'm planning to start learning the traditional forms. (I've done about 2000 simplified over two years). I'm planning to just start from here going forward; that is, not restart all my old lists from the beginning and waste my time with tons of 門，金，言，and 食 forms. I understand the traditional forms will start padding my stats for writings a bit going forward, but my question is whether if after a week or two I find this isn't working out and set it back to simplified only, will everything, including my stats, go back to the way it is now, listing only the simplified?

No; the traditional writings you've learned will still show up in your progress numbers.