NoBut here's the crux of the argument for Brown. HBHG is a "thesius" as the book has been discribed. So how would Brown useing their research for a book any different then someone else useing another research book as refrence?
The argument, as I think you're putting it, is that it's not plagiarism because the people who wrote HBHG are using other people's research too. The difference is, the authors of HBHG, i can guarantee you, cited their references.
I doubt they're asking Brown to cite on each page, just to acknowledge that he used their ideas in the book, like most professional fiction authors do when they refer to unique research in their writing.
If they are asking for more than that, then yeah, they're out of line.
ALso, the "similar character" bit is not plagiarism. Ever. There can be a case made for libel, if you can prove that he made it clear the character was supposed to resemble a real person and that there was something demeaning to his character that was false included. But simply having a character resemble a real world person has nothing to do with plagiarism.