There's an excellent article here entitled "The Unnatural History of Tolkien's Orcs" which gives the answers to your questions 2. and 3. as clearly Tolkien. Quoting from it:
Tolkien, in the creation of orcs, was very literally making up his own monsters. Tolkien also used the concepts of elves, hobbits, dwarves, and medieval-type kingdoms in creating his Middle-Earth. However, all of those concepts, or their symbolic equivalents, were strongly established in European myth and fantasy writing before Tolkien began his own works. There was no exact equivalent to the orcs. European folklore has small creatures of evil - bogies, tommyknockers, goblins - and fearsome monsters, from Grendel as commemorated in Beowulf to the evil elementals of Celtic tradition. But there was no faerie or supernatural evil that was the parallel of human warriors, which faced them as equals and was sent out to mow them down.
Tolkien needed to create his particular monsters, and their newness had a purpose. Long-term Tolkien scholar Tom Shippey has said of Tolkien's orcs, "There can be little doubt that orcs entered Middle-Earth originally just because the story needed a continual supply of enemies over whom one need feel no compunction." Hence, for the purposes of his own narratives, Tolkien combined items from then-obscure folklore with more modern concepts of violence and evil to concoct orcs. Heroes were more heroic with orcs to slay; with orcs at their bidding, higher-caste villains were more fearsome. For these purposes, orcs were a successful narrative device - and one not seen previously in fantasy writing.
Orcs were more than a good idea. There was authorial work required to set the stage for this new creation, especially to do so in a way that helped the reader support the concept. Tolkien did this so well that, in the process, he created a new archetype that became popular.
For the answer to question 1, as Richard said in a comment,
the Wikipedia article is very helpful. In summary, the word goes back to Old English "orc" and Latin "Orcus", the latter being the Roman god of Death. It has also been used in many fairy tales throughout history - interestingly, not always to denote evil creatures. And the word even has a long history in foreign languages such as Italian.
On Tolkien's use of the word:
The modern use of the English term orc to denote a race of evil, humanoid creatures has its inception with J. R. R. Tolkien. Tolkien's earliest Elvish dictionaries include the entry Ork (orq-) 'monster', 'ogre', 'demon', together with orqindi 'ogresse'. Tolkien sometimes used the plural form orqui in his early texts.
Tolkien sometimes, particularly in The Hobbit, used the word goblin instead of orc to describe the creatures. He notes that 'orc' is 'usually translated' as 'goblin'. [...]
On the ultimate origin of the word:
The word ultimately comes from Latin Orcus, the demonic Roman god of death, who should not be confused with Pluto, the god of the underworld, and has transformed by several stages from the meanings 'underworld', 'hell', 'devil', 'evil creature' to 'ogre'. Tolkien and the lexicons he used attributed the origin of the doubtful Old English orc to 'Orcus' and in one of his invented languages, the word for orc also had the form orco.
The Latin: Orcus is glossed as "Old English: Orc, þyrs, oððe hel-deofol"[a] as given in the first Cleopatra Glossary (10th century), and on this entry Thomas Wright wrote, "Orcus was the name for Pluto, the god of the infernal regions, hence we can easily understand the explanation of hel-deofol. Orc, in Anglo-Saxon, like thyrs, means a spectre, or goblin."
(Yes, I know the article is contradicting itself on whether or not Orcus was the same as Pluto! That's Wikipedia for you.)
On the word "orc" in literature:
[In] Beowulf, orcneas [are] one of the tribes of creatures named alongside elves and ettins (giants) that have been condemned by God.
The Oxford English Dictionary refers to orke, used in 1656 in a way that is reminiscent of giants and ogres. It is presumed that 'orke'/'ogre' came into English via continental fairy-tales, especially from the 17th-century French writer Charles Perrault, who borrowed most of his stories and developed his 'ogre' from the 16th-century Italian writers [...] In at least a dozen or more tales, Basile used huorco, huerco or uerco, the Neapolitan form of orco [Italian] 'giant', 'monster', to describe a large, hairy, tusked, mannish beast who could speak, that lived away in a dark forest or garden and that might capture and eat humans, or be indifferent or even benevolent — all depending on the tale.
The first English use of 'orke', in 1656 (forty-one years before Perrault published his Mother Goose tales), comes from Don Zara, a fairy tale by Samuel Holland. [...] A monster called Orcus is mentioned in Edmund Spenser's Faerie Queene (Book II, Canto XII, line xlvii).
Orc is also a proper name for one of the characters in the complex mythology of William Blake. Unlike the medieval sea beast, or Tolkien's humanoid monster, Blake's Orc is a positive figure, the embodiment of creative passion and energy, and stands opposed to Urizen, the embodiment of tradition. He is, however, publicly perceived to be demonic in nature.
On the word "orc" in languages other than English:
Words derived from or related to the Italian term orco exist in other countries: in addition to Italian dialectal uerco, huerco and huorco and the Spanish word güercu, there is also Tyrolean ork, 'a house gnome' or 'a mountain spirit' that acts as a protector of wildlife.
