-- When the teachings of Buddha are crystal clear i.e 1+1=2, then no need to be labelled.
You are right, but people have different levels of prajna (deep intuitive understanding of how things work). Someone who is very mature, because of the work done earlier in this life or in previous lives, may not need any explanations, but someone who is just starting or very confused - may need lots of words and labeling. It depends on the student.
--When buddha gave diksha, what he did: is that was the imparting of knowledge?
Not all teaching is given in words. Some teaching is given in a more subtle form. The famous Flower Sermon is a legend of how Zen Buddhism started when Buddha gave what you might call "shaktipat" to his student Mahakashyapa.
--was that the imparting of knowledge?
It is a kind of information, yes - but not in the coarse form that we usually call "information". It is a subtle kind of information that we can also call "abstract energy". So yes, it is knowledge - but it is deep intuitive kind of knowledge, not the superficial knowledge conveyed in words.
--And in the path of Buddha, once known, is to be walked upon by individual: then, how the teacher is useful? Whether he pushes us ahead by using his jhanic powers, or resolves our difficulties?
It's a little bit of everything. Also, depends on the student and on the teacher of course. But usually it has elements of 1) behavioral training, 2) leading by example, 3) regular logical knowledge, 4) emotional inspiration,
5) deeper intuitive understanding, 6) dealing with real-life situations, 7) connecting the student with external sources of energy/information, 8) meditation.
--Because in Hinduism there is mention of shaktipat by guru. Is it the same in Buddhism also?
What you call "diksha" is called Refuge in Buddhism - and yes in that ceremony has an element of "shaktipat":
At that particular point, the energy, the power, and the blessing of basic sanity that has existed in the lineage for twenty-five hundred years, in an unbroken tradition and discipline from the time of Buddha, enters your system, and you finally become a full-fledged follower of buddhadharma. You are a living future buddha at that point.
In Tibetan Buddhism there is also "abhisheka" ceremony (empowerment), which is almost completely "shaktipat".
So yes, generally speaking there are some parallels, although there are differences too - the main difference being that in Buddhism we tend to be more "scientific" and much less respectful of what you would call Sacred Elements. In Buddhism we liberate ourselves using something like "mind science", so in this sense Buddhism is more modern.
Anyway, the role of the "teacher" is not as much "to teach" knowledge, as it is to teach the skills, mature the student's mind and deliver the student to the enlightened perspective. And yes, it involves transmitting something that cannot be explained in words. Although, you must understand that in Buddhism we do not have dualistic notion of "special energy" that is somehow different from normal stuff - we just say that this energy is a subtle/abstract aspect of regular things.
But yes, just like the law systems of different countries have similarities - because they are based on the actual human nature which is more or less the same everywhere (in terms of basic goodness, vices, desires, problems) -- similarly the different religions like Christianity, Hinduism, Buddhism etc. have many common elements and many overlaps - because they are reflections of True Dharma - the True Nature of Reality.