A few questions about Middle Earth (knights, titles and names)

[dancing spawn of ungoliant]

I'd say it's a girl name because of the 'eth', I think that's a feminine ending.

At least in the name Elbereth it would seem to be a feminine suffix.
The only names (I think...) in M-E beginning with "i" and including "z" are Inzilbêth, Inziladûn and Imrazôr the Númenórean and they are all Adûnaic names.
So Izelèth would sound fine if you changed the prefix "Iz-". I'm not sure, how common an "è" would have been in the last syllable of an elvish name but with another prefix that name could mean star-lady or star-maiden or something.

About the titles...I'd add marshal on the list (for example Èomer being one of those) and lieutenant if that counts.