So much to say, but let me start by acknowledging that I'm not drawn to mantra repetition myself, and I can't comment on its efficacy, so far as inducing happiness. I'll take your word for that.
First, may I offer a set of notes on the Pali Suttas--talking here about the first four Nikayas, as the fifth is apparently of later composition and may not accurately represent the teachings of Gautama: http://zenmudra.com/Making Sense of the Pali Sutta--the Wheel of the Sayings.pdf
Second, regarding the origin of what came to be known as Mahayana Buddhism, I rely on A. K. Warder's "Indian Buddhism":
"We seem led to the conclusion that the two parties (the Sthaviravada and the Mahasamgha) were less far apart than at first sight they appear to be, except on the first ground (the five grounds: 1) that an arhant can be seduced by another person; 2) that an arhant may be ignorant of some matters; 3) that an arhant may be in doubt; 4) that an arhant may receive information [be instructed by] another person; 5) that one may enter the Way as the result of spoken words--both parties restricted 2-4 to dharma matters, so the answer was no, and although it's not clear to me from Warder's expansion it would appear the answer to 5 was also no for both parties). The Sthaviravada were categorical that an arhant was by nature beyond the reach of any possible seduction; the Mahasamgha allowed an arhant to be seduced in a dream. Between these two opinions no compromise could be found, despite all the Buddha's injunctions (in the Vinaya) on the reconciliation of dissident views.
.... No compromise having been reached, the two parties separated and became two schools of Buddhism. ... (The Mahasamgha) ... having relaxed or at least not made more stringent the conditions for an arhant, found it desirable to make a clear distinction in the case of Buddha; he was a being of quite a different nature, far above other human beings or perhaps not really a human being at all. They thus began that transformation of the Buddha, and his doctrine, which led step by step to the Mahayana, from the humanism of the original Tripitaka to the supernaturalism of most of the Mahayana sutras." (pg 217-218)
Third, as to the teaching in the first four Nikayas, having once read all the volumes, it's possible to piece together the core of the teaching. Of course it has to do with suffering, but here's a declension of the origin of suffering that makes more sense to me than most, provided the description of "birth, decay and death, grief, lamenting, suffering, sorrow, and despair" is understood to be "in short, the five groups of grasping" (AN I 176, Pali Text Society Vol I pg 160):
"That which we will…, and that which we intend to do and that wherewithal we are occupied:–this becomes an object for the persistance of consciousness. The object being there, there comes to be a station of consciousness. Consciousness being stationed and growing, rebirth of renewed existance takes place in the future, and here from birth, decay, and death, grief, lamenting, suffering, sorrow, and despair come to pass. Such is the uprising of this mass of ill.
Even if we do not will, or intend to do, and yet are occupied with something, this too becomes an object for the persistance of consciousness… whence birth… takes place.
But if we neither will, nor intend to do, nor are occupied about something, there is no becoming of an object for the persistance of consciousness. The object being absent, there comes to be no station of consciousness. Consciousness not being stationed and growing, no rebirth of renewed existence takes place in the future, and herefrom birth, decay-and-death, grief, lamenting, suffering, sorrow and despair cease. Such is the ceasing of this entire mass of ill."
(SN II 65 “Kindred Sayings on Cause” XII, 4, chapter 38 “Will”, Pali Text Society vol. 2 pg 45)
The teaching is about the cessation of intentional or volitive activity, and Gautama taught that this occurs gradually as the meditative states unfold. In particular and necessary to the mindfulness he described as his "way of living" (SN V 320-322, Pali Text Society SN V pg 285) was the cessation of action of the body in the fourth of the rupa jhanas, and the "survey sign" of the concentration (ok, that would be my opinion, but it's very true--as my father used to say).
How is it possible, to sit and relinquish willful activity in the body? What's it like when action of the body takes place involuntarily, as though a part of the movement of breath?
Here's Kobun Chino Otogawa (who came from Eiheiji to help Shunryu Suzuki found Tassajara Monastery):
It’s impossible to teach the meaning of sitting. You won’t believe it. Not because I say something wrong, but until you experience it and confirm it by yourself, you cannot believe it.
(“Embracing Mind”, edited by Cosgrove & Hall, pg 48)
And that would be why it's hard to make sense out of Gautama's teachings, but you know there's this:
“…What do you think about this, reverend Jain: Is King Seniya Bimbisara of Magadha, without moving his body, without uttering a word, able to stay experiencing nothing but happiness for seven nights and days?”
“No, your reverence.”
… “But I, reverend Jain, am able, without moving my body, without uttering a word, to stay experiencing nothing but happiness for one night and day. I, reverend Jain, am able, without moving my body, without uttering a word, to stay experiencing nothing but happiness for two nights and days, for three, four, five, six, for seven nights and days.”
(“Cujadukkhakkhandhasutta”, MN I 94, Pali Text Society Vol I pg 123-124)
