Concerning the word
wrath, I don't think it was an intentional case of having a Wizard and a Man of Rohan pronouncing a word differently to 'shake things up', so to speak.
Perhaps I am wrong but I have always had the impression (and my somewhat unreliable dictionary agrees with me!

) that
rath is an incorrect pronounciation of the word. To add to this, from a 'booking' point of view, I think it made King Theoden look bad to have Gandalf practically correct him a few minutes later.
But Bob knows I am as fallible as they come so please don't hesitate to correct me if I am mistaken.