Can the teaching be reduced to Paticca Samuppada? Has lord Buddha preached so in any Sutta? During vipassana meditation, the practitioner focuses on impermanence, dissatisfaction and non-self. After this, when getting Arhantship, is it Paticca Samuppada that is understood by the disciple?
The Buddha's Dharma is beautiful in the beginning, beautiful in the middle, and beautiful in the end. Absent need for addition or reduction. It would be overly simplistic to reduce the entire teaching to one doctrinal aspect - even if it were dependent origination.
Having said that - one who sees dependent origination, sees the Dharma. One who sees the Dharma sees dependent origination. Without realizing the knowledge behind paticcasamuppada, it would not be possible progress to sainthood. For even Sotapannas need to see the Dharma in order to eradicate the first three fetters.
Rather than postulating what it is that an arahant realizes at the point of fruition that successfully eradicates the asavas, focus more on the cultivation of morality and concentration. This will arm you with the tools you need to investigate phenomenons for yourself. As your paramitas mature, you will realize these truths progressively.
Can the teaching be reduced to Paticca Samuppada? Has lord Buddha preached so in any Sutta?
Sariputra said (MN 28):
Now, the Blessed One has said, whoever sees paticcasamuppada sees the Dhamma; whoever sees the Dhamma sees paticcasamuppada.
Unfortunately, Sariputra's quote is the only place this phrase occurs in the entire Canon, so we can't see its meaning from the context Buddha has said it in - as we can and should with most canonical phrases.
In another place (MN 22) Buddha said:
Both formerly and now, monks, I declare only dukkha and the cessation of dukkha.
-- although this was said in context of polemics with "some brahmanas and shramanas" who misunderstood nirvana as "annihilation, destruction, extermination of the existing being".
In Uposatha Sutta (KN Ud 5.5) Buddha said:
Just as the great ocean has one taste, the taste of salt, so also this Dhamma-Vinaya has one taste, the taste of liberation.
When getting Arhantship, is it Paticca Samuppada that is understood by the disciple?
From many, many places in other texts and commentaries, it is apparent that the gist of Dharma are the four noble truths and the three marks of existence. In my own words, liberation (cessation/prevention of dukkha) is attained on the basis of seeing things as they are (second noble truth / idampratyayata, three marks of existence, pratityasamutpada) which plays the role of causing disenchantment which plays the role of preventing the arising of thirst.
This said, I agree with Amanasa - I think that trying to boil down Buddha-Dharma to a single concept is dangerous and outright wrong. Instead, from my personal experience implementing it in practice, it feels more like falling out from all concepts and being able to juggle them as needed. It feels like having no home in any position or philosophy, no ground under your feet. Amanasa is right in advising to focus on cultivation of morality -- and in Mahayana cultivation of morality goes hand in hand with deconstructing the ego:
At the beginning, this process is very coarse. There is a huge obvious difference between egoistic and altruistic behavior. As you proceed forward however, you start noticing more and more ambiguous situations. What looked like an altruistic act on the surface, quite often turns out to be a hidden move for preservation of one's self-image.
At this early stage we must stop trusting our own thoughts, and get in the habit of critically examining our motives. This change in attitude to self marks the turning point when consumer turns into practitioner.
At this next level, ego often manifests as "spiritual materialism" -- a subconscious tendency to accumulate goodness as food for one's ego, to get a kick out of being "perfect" -- a secretly perverted spirituality driven by a hidden motive of self-aggrandizement. One may easily get as far as acting egoless as a way of feeding one's ego! Now our game must get a lot more subtle. Practitioner has to learn to be a hunter, quietly stalking its pray -- the ego playing its tricks.
When you're doing this right, you will eventually find yourself intentionally acting "bad", as an antidote for spiritual materialism. (It is a lot subtler than regular uncontrolled misbehavior though, and its motive is very different.) And guess what, as you go further, you may realize that this too could be an ego's trick, at which point you double up and get back to being good, now with 8 times less ego flavor than originally!
At some point, when the process of doubling up your mindfulness upon itself gets to its 16th or 32nd degree (or 64th, or 128th etc. depending on a person), the game becomes too subtle to maintain. The whole thing explodes, destroying any sense of division into "observer" and "observed".
This is what I would recommend to focus your practice on, stalking and starving the ego. Once ego gets thin enough, realization of theoretical concepts (like pratityasamutpada) comes easy.