David, the questions you have raised are at the heart of understanding the keris. However, to address those questions we must place them within a framework that encompasses time and place.
In my "Interpretation" article I addressed the Naga and the Singo in terms of pre-Islamic Jawa, and to a degree also in terms of pre-puputan Bali. You can see what I wrote here:-
http://www.kerisattosanaji.com/INTERPRETATIONPAGE4.html
Following the domination of Jawa by Islam the structure of society changed and the way in which the Hindu-Buddhist iconography was understood also changed. In my opinion there seems to be a consistent inconsistency in the way in which much of keris iconography is understood across the various societies that we can identify as "keris bearing societies".
When we consider the keris in terms of pre-Islamic Jawa we are dealing with a society and culture that existed within a well defined framework. The Hindu-Buddhist period in Jawa has been extensively studied over many years and we have a good body of work to draw upon that permits us to form opinions that can be defended. We can construct hypotheses, and perhaps even theories, and whether or not those hypotheses and theories are correct, we are able to establish a foundation that provides a beginning to the gaining of an understanding of the keris as a symbol of a culture.
However, when Islam replaced the Hindu-Buddhist belief system, the rules changed, and those changes have continued until today. To respond to your questions in simplistic terms we can go and have a look at one or another of the multitude of keris books that we now have available, and we can either accept or reject what we read there. If we do not particularly like what we read, we can go to a different book, and continue thus until we do find something that we can agree with.
In present day Javanese society it seems as if the Singo Barong is more or less generally accepted as a symbol of power, authority and resolution, if found on a keris it indicates the hope of the bearer to possess these qualities.
In present day Jawa the Naga appears to be more or less generally accepted as a symbol of power and wisdom, and if found as a motif in a keris, then that expresses the hope of the bearer to possess these qualities.
So, Singo Barong:- power, authority, resolution.
Naga:- power , wisdom.
Put them together:- power, authority, resolution, wisdom.
Who needs this?
Well, everybody I guess, but such qualities are particularly relevant to leaders and rulers. Maybe if a night-bus driver has dreams of becoming head of his company, or head of his village, or head of his nation, the first thing he needs to do is take himself down to his local market and buy a keris naga, keris singo barong, or better yet, a naga and a singo barong neatly mated in the one keris.
But this symbolism expresses a hope, not necessarily a reality, so what we then have is the present day interpretation of the keris as :-
"A Prayer in Steel".
Which could be understood as an Islamic interpretation of something that a Hindu-Buddhist in 13th century Jawa might have interpreted in a somewhat different way. When Islam set out to re-organise Javanese society, it followed the soft road of conversion by taking existing symbols and beliefs and gently and gradually changing those symbols and beliefs into ideas that were more acceptable to Islam.
For those of us outside the relevant societies, the way in which we understand the keris becomes a matter of understanding the people who have the keris as a part of their societal and cultural fabric, and attempting to come to terms with the way in which those people understand or understood the keris, within a defined framework. Our own limited understanding is secondary.