Hmm, can you give an example of when it could be understood to modify "Fitzwilliam"? I don't see it at all. Not without completely changing the sentence . . .
I'm curious what you see.
(And welcome to AW!)
Good to be here!
The confusion is mainly because we're dealing with semantics, not just grammar.
Who is a pronoun with an antecedent, but which one depends on context.
Suppose the example is the response to a question: "What is the name of a senior partner at Dewey, Cheatham, and Howe?" The answer is
Fitzwilliam/one (they're interchangeable), and
who resolves to that. The other information is less important, that is, you could answer, "Fitzwilliam," and satisfy the question. To put it another way, the information about seniority applies to Fitzwilliam as an individual, while
of those lawyers only identifies him as a member of a certain group, so you could leave out that phrase.
Now suppose there was a different question: "Has that guy been here long?" The answer supplies his name and profession, but that's not the information requested. You can't answer, "Fitzwilliam," and satisfy this question. We want to know if
that guy is a senior partner, so the information that modifies
lawyers is more important.
If that's not clear enough, we can try something else.
By the way, in a previous life I was a professional copyeditor. That means a) I know English grammar extremely well, b) I don't take it personally when someone disagrees with me or shows where I'm wrong, c) I know there are cases where there is no single correct answer, and d) the editor is always right.