Good observations. A few additions ...
First, they're Wh-words, not just W-words. The wh part goes back to Proto-Indo-European relative/interrogative root *kʷo- and shows up in Latin as well as in all the qu-words and -phrases we've borrowed, like quantity, quality, and question.
Second, you missed a few. As well as who, what, where, when, why and how, there's also which and whether. Not to mention old paradigmatic variants like whence and whither, which are pretty rare in use these days.
Third, some is not the only word they can occur with. Consider any, which alternates with some in many contexts:
- Did you see some people there? ~ Did you see any people there?
So besides somewhere, somehow, and somewhat, there's also anywhere and anyhow. And then there's everywhere and elsewhere. It looks like locatives have the most fun, since where combines with the most words. Combinations like *somewhen, *anywhat, *elsewho, and *everyhow -- while understandable -- just don't exist in English.