I'll try to answer your first question about the connection between etymological meaning and philosophy:
I can't see how the etymology of a word has any significance other than an historical significance.
I am with you on this point.
However, there is a well known tradition in philosophy that did hold the opposite view in high regard: the etymological meaning is actually the most relevant one for philosophy!
So how can we characterize this connection between the original formation of a word and its current, or derived, use? (For instance "energy" is a word that has all but completely lost its Aristotelian meaning as energeia.)
As far as I know, the tradition advocating the view "etymology matters philosophically" held that this connection (what we call evolution of language) was a successive series of obfuscation and distortions. It is a form of linguistic essentialism.
Martin Heidegger probably was the most important proponent of this tradition, as he thought that ancient Greek had a special philosophical significance:
Here a statement of principle is required. If we listen now and later to the words of the Greek language, then we move into a distinct and distinguished domain. Slowly it will dawn upon our thinking that the Greek language is no mere language like the European languages known to us. The Greek language, and it alone, is λόγος. We shall have to deal with this in greater detail in our discussions. For the time being let it be sufficient to suggest that in the Greek language what is said in it is at the same time in an excellent way what it is called. If we hear a Greek word with a Greek ear we follow its λέγειν, its direct presentation. What it presents is what lies immediately before us. Through the audible Greek word we are directly in the presence of the thing itself, not first in the presence of a mere word sign. (Heidegger, What is philosophy?, p. 45)
That is, Heidegger claims that there is an essential connection between things (concepts?) and word signs in the Greek language (that other languages lack). Note that he speaks of "audible Greek word", i.e. he includes sounds. The Greek language is transparent, the language itself shows its meaning somehow...
This clearly goes against the intuition that language and linguistic meaning are just glued together by convention. As Shakespeare famously put it:
What's in a name?
That which we call a rose
By any other name would smell as sweet.
(Romeo and Juliet II, ii, 1-2)
So, where does that leave us with Heidegger's claim?
Interestingly, 20th century linguistics has given an interesting twist to some of the most speculative (philosophical) ideas by reformulating them in a naturalized manner.
In this vein Plato's innatism (the claim that humans have access to certain inborn ideas) has been revived by Noam Chomsky and is today the standard explanation of how children can acquire language skills that fast. (See also Plato's Problem.)
Similarly, the claim that the connection between linguistic meaning and language is in some cases not conventional has been revived in linguistics and forms the basis of a whole field of investigation called Phonosemantics. The successfully tested hypothesis here is that vocal sounds have meaning and, in particular, that phonemes (the "building blocks" of any language) have meaning. This is not really Heidegger's original claim (in the same way that Chomsky's contribution is a long shot from Plato's original conception), but is an interesting revival nonetheless.
Still, this just shows that the connection between natural languages and linguistic meaning is not conventional in some cases. It doesn't provide any argument as to why this meaning might be relevant to philosophical reasoning.