1. We've actually dealt with your first question in another thread. Have a look at it as it contains a discussion of other instances where the article is omitted, which might interest you:
http://www.eslhq.com/forums/esl-forums/english-questions/use-definite-article-53544/. It's common for the definite article to be omitted with professional titles or positions - eg
Barack Obama is (the) President of the United States. What I said in that thread was :
"This type of sentence is typical of journalistic genres - a noun phrase in apposition with (= juxtaposed with and equated to) the name, sometimes with the definite article omitted, sometimes not. Here are some more examples which I found in 2 mins just browsing the BBC News website today :
a) With omission of the definite article
Film critic Roger Ebert said the actor had come across as "sad and pathetic".
Martin Johnson, director of the Thalidomide Trust, told the BBC that ....
Mining Minister Rafael Ramirez said operations could resume within two days of the site being declared safe.
The fighting came as Syrian Prime Minister Wail al-Halqi met Iranian Supreme Leader Ayatollah Ali Khamenei and President Mahmoud Ahmadinejad.
b) With the definite article included
The lead researcher Dr Angela Attwood told the BBC...
As you can see, the omission is far more common than the inclusion."
Notice that it's the definite article though, not the indefinite article. the sense of your sentence is : Galadriel and Celeborn are (the) rulers of Lothlorien. Hence your original sentence, with the article added would read :
The Lady Galadriel is the ruler, along with etc. Using the indefinite article would suggest an indefinite number of rulers, whereas the sentence is telling us it's a definite quantity - two.
2. The definite article is used with the title "lady" when describing the person. Here are some examples from the press : a) the Daily Telegraph -
The Oscar winner said that even though the Lady Thatcher's views were shared by many of her male colleagues in government they escaped ... b) the ITV news website -
A handbag carried by the Lady Thatcher, raised £25,000 at auction last year. c) the Daily Mail -
as every good leftist knows Al Gore was much more honorable than the Lady Thatcher.
The article is often dropped when people are speaking informally though, and I suspect that most people who don't spend time with such illustrious company don't even know the rule. Here's what one writer said :
"Lady Thatcher" is appropriate for reference in conversation, with the more formal styles being correct for correspondence and legal documents, and ....formal occasions. So for me, Tolkien's use of the article gives the text a rather archaic and poetic feel, which adds to the etherial "elven" image of Galadriel.
3. Yes - it would be more normal.