As always, if it's of any use here...
From the same Wiki... Western scholars have not reached a consensus on the origin of the reverence for Avalokiteśvara.
I guess the answers may eventually tie back to the very foundation question of what Mahayana is...or isn't
If I may recommend reading firstly, Brahmanism, Buddhism, and Hinduism
then Paul William's MahÄyÄna Buddhism: The Doctrinal Foundations
in his presentation of the origins of Mahayana from page 21 onwards to relook at the claim of '...that Guanyin is an incorporation of Hinduism into Buddhism'
And this curious one...
http://www.dharmaweb.org/index.php/Aval ... Compassion
Avalokitesvara first appears in Indian Buddhism. He is originally mentioned as one of a number of bodhisattvas. These bodhisattvas are personifications of various attributes of the
Buddha. Avalokitesvara is the personification of compassion. The development of a Buddhist doctrine of bodhisattvas is more or less contemporaneous with the development of brahmanic deity worship. Either the same societal forces led to both developments, or the bodhisattva doctrine was a response to the rival movement of brahmanic deity worship. The bodhisattva doctrine may have appeared as early as the second century B.C.E.
Bodhisattvas originally appear as attendants of the buddhas. Texts speak of there being vast numbers of bodhisattvas. A few of the bodhisattvas are more important than others. Avalokitesvara does not appear in the earliest texts about bodhisattvas. However, after a while he becomes one of the important bodhisattvas.
By the second century C.E., in the larger Sukhavativyuha, Avalokitesvara is described along with Mahasthamaprapta as one of the two bodhisattvas in Sukhavati, the pure land of the Buddha Amitayus. The two of them are described as the source of the light that illumines the pure land. They also teach the devotees of Amitayus, adapting their techniques to the understanding of the listeners. Avalokitesvara's rise in prominence did not stop at this point.
Probably around the fifth century C.E., a full-blown cult of Avalokitesvara emerged. Avalokitesvara evolves into the supreme savior of all suffering beings. He takes on the characteristics of various brahmanic gods, such as Brahma, Visnu, and Siva. Like Brahma, Avalokitesvara is described as the creator of the universe.
"From his eyes arose the sun and the moon, . . . from his mouth, the wind, . . . from his feet, the earth." He is also described as being the creator of the brahmanic dieties.
This attribution of power to Avalokitesvara may well have been aimed at proselytizing among brahmanic followers.
Prof Lokesh Chandra offers an idea here
Otherwise look online for works from:
Pachow, W. “The Omnipresence of Avalokiteśvara Bodhisattva in East Asia.” Chinese Culture Quarterly 28.4 (1987): 67–84.
Looks at a general survey of Avalokitesvara's prominence in Asia via historical and literary sources
1. “Celestial Buddhas and Bodhisattvas.” In Encyclopedia of Religion. Vol. 3. Edited by Mircea Eliade, 134–143. New York: Macmillan, 1986.
Explores a concise definition of Buddhas & Bodhisattvas in Mahayana and how they differ from Early Buddhism via what he uses the tern 'celestial'
2. "Indo-Tibetan Buddhism: Indian Buddhists and Their Tibetan Successors." 2 vols. Boston: Shambhala, 1987.
New look and understanding on the cults that surrounds and are devoted to the Buddhas & Bodhisattvas
Tay, C. N. “Kuan-yin: The Cult of Half Asia.” History of Religions 16.2 (1976): 147–177. Reissued in 1987
First exploration in English on Guan Yin via scriptures and commentarial literature, mainly focusing on China but also much of East Asia
For a more scriptural look: Try this
Probably as some would opine, the earliest mention on Avalokitesvara would be from the Saddharmapundarika Sutra
, dated between 100BC-100AD and latest by 200AD where in the most revered & popular Chapter 25 of Kumarajiva translation to Chinese (from which the first English version came from Hendrik Kern in 1884 and subsequent ones) has it dedicated to Avalokitesvara.
There's a thread on Amitabha here
Avalokitesvara less real? What is 'real' to anyone these days? As how a Buddha, Nagarjuna or Asanga opined it?
I would ask myself if Bodhicitta, which has the facet of Great Compassion, is less 'real'? Is the Bodhisattva Path & the career of a Samyak Sambuddha less 'real'?
Who knows? You may be visited by Guan Yin in a cup of tea or on the cushion or when offering the less fortunate your sandals / coat or the flash of lightning or a dew drop?
Think about it...don't miss Him in every moment of life...Who sees me in forms,
Who seeks me in sounds,
He practices a deviant way,
And cannot see the Tathagata