Nice Question. This post is to attempt to answer 2 of your questions in context to Vedanta (you may infer the rest from the below answer too) -
The same question is posed by Yudhistira to Bhishma (in a different form). This can be found in Chapter 162, Anushasana Parva, Mahabharata. I have pasted the relevant sections below (providing only relevant text) but i urge you to read the entire chapter for contextual understanding.
"Yudhishthira once more asked Bhishma the son of Santanu,
saying, 'O thou of great intelligence; O foremost of all persons
conversant with duties, which, indeed, of the two, direct perception
and the scriptures, is to be regarded as authority for arriving at a
"Bhishma said, 'I think, there is no doubt in this. Listen to me, O
thou of great wisdom! I shall answer thee. The question thou hast
asked is certainly proper. It is easy to cherish doubt. But the
solution of that doubt is difficult.
Innumerable are the instances, in respect of both direct perception and audition (or the scriptures), in which doubts may arise. Certain persons, who delight in the name of logicians, verily imagining themselves to be possessed of superior wisdom, affirm that direct perception is the only authority.
They assert that nothing, however true, is existent which is not directly
perceivable; or, at least they doubt the existence of those objects.
Indeed, such assertions involve an absurdity and they who make them
are of foolish understanding, whatever may be their pride of learning.
If, on the other hand, thou doubtest as to how the one (indivisible
Brahman) could be the cause, I answer that one would understand it
only after a long course of years and with the assistance of Yoga
practised without idleness. Indeed, O Bharata, one that lives
according to such means as present themselves (without, i.e., one's
being wedded to this or that settled mode of life), and one that is
devoted (to the solution of the question), would be capable of
understanding it. None else, truly, is competent for comprehending it.
When one attains to the very end of reasons (or reasoning processes),
one then attains to that excellent and all comprehending
knowledge--that vast mass of effulgence which illumines all the
universe (called Brahma). That knowledge, O king, which is derived
from reason (or inferences) can scarcely be said to be knowledge. Such
knowledge should be rejected. It should be noted that it is not
defined or comprehended by the word. It should, therefore, be
"Yudhisthira said, 'Tell me, O grandsire, which among these (four) is
most authoritative, viz., direct perception, inference from
observation, the science of Agama or scriptures, and diverse kinds of
practices that distinguish the good.'
"Bhishma said, 'While Righteousness is sought to be destroyed by
wicked persons possessed of great might, it is capable of being
protected for the time being by those that are good exerting
themselves with care and earnestness. Such protection, however, avails
not in the long run, for destruction does overtake Righteousness at
the end. Then, again, Righteousness often proves a mask for covering
Unrighteousness, like grass and straw covering the mouth of a deep pit
and concealing it from the view. Hear, again, O Yudhisthira! In
consequence of this, the practices of the good are interfered with and
destroyed by the wicked. Those persons who are of evil conduct, who
discard the Srutis--indeed, those wicked wights who are haters of
Righteousness,--destroy that good course of conduct (which could
otherwise be set up as a standard). Hence, doubts attach to direct
perception, inference, and good conduct.
Those, therefore, among the good that are possessed of understanding born of (or cleansed by) the
scriptures and that are ever contented, are to be regarded as the
foremost. Let those that are anxious and deprived of tranquillity of
soul, approach these. Indeed, O Yudhishthira., do thou pay court to
them and seek of them the solutions of thy doubt! Disregarding both
pleasure and wealth which always follow lust (desire) and awakened into the
belief that only Righteousness should be sought, do thou, O
Yudhishthira, wait upon and ask those persons (for enlightening
thyself). The conduct of those persons never goes wrong or meets with
destruction, as also their sacrifices and Vedic study and rites.
Indeed, these three, viz., conduct as consisting of overt acts,
behaviour in respect of (mental) purity, and the Vedas together