I'm not sure I understand why you couldn't find this information on the internet. After running a quick google search I found
this,
this,
this and especially
this.
I also don't know how much information you need about Structure, Tonality, and Melody etc. They can have a one word or one paragraph long answer. For example, Structure is in sonata form. How much more do you need? And if you want to compare it to another piece, why not just chose Mendelssohn's more famous overture, A Midsummer Night's Dream? Hebrides meets 10 minutes and Midsummer reaches 12 minutes long. Is that sufficient for you?
As far as the Hebrides, I think those links I mentioned above, especially the last two, should give you information for everything you listed including compositional devices. But in case you want more, here is some general background information from some of my sources (books):
From Plantinga: "This piece typicaly presents mildly illustrative musical materials (such as the wavelike melodic motion at the beginning) within very regular sonata-allegro structures."
From Hugo Leichtentritt: "[Mendelssohn] also sought to give musical impressions of landscapes. In his Italian and Scotch symphonies he succeeds in evoking in the mind of the listener the impression of characteristic local color that is suggestive of Italian and Scotch scenery, and his Fingal's Cave overture has earned for him the praise of so severe an opponent as Richard Wagner, who called Mendelssohn a great landscape painter in music with reference to this particulare overture. Certainly it gives a wonderfully vivid impression of the surging sea, of waves resounding in rocky caves, of the harsh cry of the sea gulls, the odor of the salt air, the sharp flavor of sea weed, and the melancholy soul of this northern scene."
From Men of Music: "The Hebrides overture disputes place with that to A Midsummer Night's Dream in quality. We have the composer's own testimony that the lapping figure with which it opens came into his mind during his visit to Fingal's Cave in the remote Hebrides. Wagner, with his genius for trivializing the truth, sneeringly called the overture an aquarelle. Actually, it is one of the great seascapes of music. The severity and aptness of the themes, unquestionably among Mendelssohn's happiest inspirations, the utter sufficiency of the development, and the uncanny balancing of the instruments - all contribute to a formal perfection that has, as Tovey says, 'the vital and inevitable unexpectedness of the classics.'"
These quoted paragraphs are actually minute details compared to the links I gave... but I hope at least something here helps you.
