So, I just double-checked Marion's introduction to her brother in the Greyhaven anthology -- the place where she gives him credit for helping with The Spell Sword. (The detailed blog post on Paul Edwin Zimmer's author credits can be found here.)
On one hand, she says Paul wrote completed "chapters" of text and that she splits royalties from the book evenly with him. That seems to suggest that PEZ deserves a (secondary) author credit for the book.
That "chapters" bit, though, seems like an exaggeration. At best, though only about 10-15 pages of The Spell Sword (out of 160 total) are devoted to fight scenes that would have required PEZ's expertise. And we also have the fact that, whereas Marion explicitly states that PEZ deserves an author credit for Hunters of the Red Moon, she's silent about The Spell Sword. Surely, if she felt he deserved credit on the Darkover novel, she would have said so? Especially as Marion isn't the sort of begrudge credit where it's due.
All of makes me lean against attributing The Spell Sword as one of PEZ's "nine" novels, and instead thinking Marion had placed the unpublished The King Who Was of Old among those nine.