I'm more qualified to answer your second question, and I'm mostly approaching this from the perspective of translation and intellectual culture in medieval Spain and the Christian West. I look forward to seeing if a historian of philosophy is able to tackle the first question. The first thing to understand about Averroes is that he was writing in an Andalusian context, meaning that he was in relatively close geographic and cultural proximity to Christians and Jews, and therefore Christian and Jewish learning traditions. Medieval Iberia was an active intellectual marketplace in which scholars from all three faiths wrote, copied, bought, sold, and translated manuscripts on everything from science to philosophy to magic. A good deal of this intellectual labor also involved annotating or glossing existing works to promote new interpretations or contest rival ones. The role of the translator or commentator was not so simple as a modern person might assume: translators often made major changes to the original works, including shortening or omitting passages, choosing words and phrases that did not match the meaning of the original text, and even adding lengthy passages or digressions, sometimes in their own translatorial voice, and sometimes passing these passages off as the work of the original author. In the same vein, commentators often added significant content and context to the original work by asking questions, adding examples, or framing the ideas in a new way. The act of translating or commentating could therefore be seen as the creation of an entirely new intellectual work, rather than the mere transfer of knowledge from one language or context to another.
Many of you may guess where I'm going with this. Averroes is most commonly known as "the Commentator" due to his commentaries on several of Aristotle's works. To oversimplify things, most of Aristotle's works had been "lost" in the Western Christian tradition, but had survived in Arabic translations made from the Greek in the early medieval period. Averroes's predecessors contributed to this Greek tradition significantly through their commentaries and glosses. In some ways, Averroes was behind the times for focusing so much on Aristotle (Avicenna was much more in fashion at the time), but the geographic proximity to Christian translators hungry for Aristotle made the Christian audience more receptive to Averroes, albeit desiring the Aristotelian texts themselves more than Averroes's interpretations of them, which were quite controversial.
The Western Christian tradition laid claim to a mythologized patrimony in which they were the "cultural heirs" of the classical Greeks. Scholar translators flocked to Spain and Sicily in the hopes of recovering these works and translating them into Latin, to absorb them into the Western philosophical tradition and the burgeoning university culture in Europe. These translators sought out Arabic manuscripts claiming to be lost Greek manuscripts, particularly the works of Aristotle. (I'll also add at this point that there were many manuscripts claiming to be the works of classical Greek authors, that were likely actually the works of later Muslim writers writing in Arabic). They used a few methods for translating them: some of them learned the Arabic language themselves, but most employed Mozarabs (culturally Arabized Christians who spoke Arabic), Jews who spoke Arabic, or former Muslims who had converted to Christianity to translate the works.
These scholars were very interested in "rescuing" or "salvaging" the works of the Greek authors, so they focused on gaining access to these manuscripts in any way possible. There are a few fascinating stories of Spanish cities recently conquered by Christians being inundated with scholars eager to raid the libraries of the barely-vacated palaces - a cultural facet to the reconquista. There was certainly some contemporaneous understanding of this dynamic; one twelfth-century jurist in Seville advised Muslims not to willingly give scientific manuscripts to Jews and Christians, because they translated the texts into their own languages and passed them off as their own.
Christian translators often heavily abbreviated and even ignored the contributions of Muslim commentators. Michael Scot, for instance, often abbreviated or omitted Averroes's commentary on Aristotle's Metaphysics, while leaving intact the parts of the text which he believed to be the authentic work of Aristotle. Stories like this are common, and the translators' introductions to these works offer a fascinating glimpse into their worldview. They describe attempting to preserve the true words of the Greek authors, while limiting the impurities and later corruptions of the Arabic commentators. Some even justified their choice to heavily abbreviate the Arabic original because they claimed that Arabic was an overly wordy language, and the meaning of the original philosopher's words could be divined and preserved without the need for literal, word for word translation from the Arabic.
Averroes was much more than a philosopher and commentator on Aristotle: he was also a judge, linguist, and physician, for instance. The West's reception of him, however, has been colored by his commentaries on Aristotle and his association with the recovery of Aristotle's works. I don't know enough about Islamic philosophy to know whether he is "more" or "less" significant, but the reason he is considered so significant in the West is because of the major role he played in bringing Aristotle's works to the Christian world. This was, however, not necessarily the intention of the Christian scholars, whose primary goal was to bring the Greeks into the Latin tradition so they could supersede and replace the Arabic tradition and the Islamic world more broadly.
Sources (cribbed from my thesis bibliography):
Beer, Jeanette, ed. Translation Theory and Practice in the Middle Ages. Kalamazoo, Mich: Western Michigan University, 1997.
Blumenfeld-Kosinski, Renate, Luise von Flotow, and Daniel Russell, eds. The Politics of Translation in the Middle Ages and the Renaissance. Ottawa: University of Ottawa Press, 1991.
Brentjes, Sonja, Alexander Fidora, and Matthias M. Tischler. “Towards a New Approach to Medieval Cross-Cultural Exchanges.” Journal of Transcultural Medieval Studies 1:1 (2014): 9-50.
Many, many works by Charles Burnett.
Copeland, Rita. Rhetoric, Hermeneutics, and Translation in the Middle Ages: Academic Traditions and Vernacular Texts. Cambridge, UK: Cambridge University Press, 1991.
Cotts, John D. Europe’s Long Twelfth Century: Order, Anxiety, and Adaptation, 1095-1229. New York: Palgrave Macmillan, 2013.
Gutas, Dimitri. “What Was There in Arabic for the Latins to Receive? Remarks on the Modalities of the Twelfth-Century Translation Movement in Spain.” In Wissen über Grenzen: Arabisches Wissen und Lateinisches Mittelalter, edited by Andreas Speer and Lydia Wegener, 3-21. Berlin: de Gruyter, 2006.
Hasse, Dag Nikolaus. “Abbreviation in Medieval Latin Translation from Arabic.” In Vehicles of Transmission, Translation, and Transformation in Medieval Textual Culture, edited by Robert Wisnovsky, Faith Wallis, Jamie C. Fumo, and Carlos Fraenkel, 159-172. Turnhout: Brepols, 2011.
Hollengreen, Laura H., ed. Translatio, Or the Transmission of Culture in the Middle Ages and Renaissance: Modes and Messages. Turnhout: Brepols, 2008.
Mews, Constance J. and John N. Crossley, eds. Communities of Learning: Networks and the Shaping of Intellectual Identity in Europe, 1100-1500. Turnhout: Brepols, 2011.
Wisnovsky, Robert, Faith Wallis, Jamie C. Fumo, and Carlos Fraenkel, eds. Vehicles of Transmission, Translation, and Transformation in Medieval Textual Culture. Turnhout: Brepols, 2011.