When it comes to single English dictionary words, what exactly counts? Is it an entry in either the American and British dictionaries, are both required, or one of them in particular? And are newly added to dictionaries words counted?
As always, depends. Plenty words to register today in any dictionary (American, British, whatever), but it's pretty pointless if nobody knows what the word is. It's a common misconception when people hear that one-word dictionary words are valuable. They certainly can be, if they're understood, and not just any random crap (especially a new / unknown word). As a Brit, if you asked me would I prefer words from the American or British dictionary, I'd have to say American. The world turns to the American way far more, considering size / dominance, equaling a larger pool of end users. Not to say British dictionary has small value, of course it doesn't. I only mean this generally, given what I said about buyers
If the official spelling of a dictionary word is hyphenated, is the non-hyphenated domain still vastly preferred and likely more valuable?
Again, depends. A lot depends on your end user too. Take for example six-pack. Would I take the technical six-pack over six pack. I'd take non hyphen, personally. Anti-theft over anti theft. Again, if given the luxury of choice, I'd take non hyphen. However, hyphen is appropriately placed in both examples, and still considerably valuable