Is Shanti Parva an interpolation?
Mahabharata itself says that originally it had 24,000 verses exclusive of the episodes. Only later there was another compilation that extended it to 100,000 verses.
Vyasa executed the compilation of the Bharata, exclusive of the
episodes originally in twenty-four thousand verses; and so much only
is called by the learned as the Bharata. ............. After that he
executed another compilation, consisting of ...one hundred thousand in
the regions of mankind.
Mahabharata, Adi Parva, Section I translated by Sri K. M. Ganguli
This pretty much states that the majority of the text we see today was a later addition. So while there may have been a Shanti Parva in the first version of Mahabharata (Shanti or peace must come after the 18-day war) there is no doubt that a lot of the material in the Shanti handed down to us is a later addition.
Does all this matter? Does Hinduism depend on whether the Shanti or any other parva has interpolations? Does Bhishma speak only banalities in the Shanti?
The answer is a firm no to all three. Mahabharata is not a source text of Hinduism. The Vedas and particularly the Upanishads are the source texts. The ten Upanishads commentated upon by Shankaracharya and the Upanishads mentioned by him in the commentary are accepted by scholars as authoritative.
Upanishads are the most important sources of Vedanta philosophy. The
ten Upanishads on which Sankara (A.D. 788-820) wrote commentaries have
been accepted as more ancient and authoritative. Mandukyoupanishad,
the smallest of these, has the unique distinction of being commentated
upon by Gaudapada (7th cent. A.D.), the teacher of Govindapada (8th
cent. A.D.) who was himself the teacher of Sankara.
A concise encyclopedia of Hinduism by Swami Harshananda
What is the role of Mahabharata?
The wisdom of this work, like unto an instrument of applying
collyrium, hath opened the eyes of the inquisitive world blinded by
the darkness of ignorance. As the sun dispelleth the darkness, so doth
the Bharata by its discourses on religion, profit, pleasure and final
release, dispel the ignorance of men. As the full-moon by its mild
light expandeth the buds of the water-lily, so this Purana, by
exposing the light of the Sruti hath expanded the human intellect. By
the lamp of history, which destroyeth the darkness of ignorance, the
whole mansion of nature is properly and completely illuminated.
Mahabharata Adi Parva Section I translated by Sri K. M. Ganguli
Has Mahabharata lived up to its promise? I think the answer is yes. A short section of the Mahabharata, Gita, has reached the status of one of Prasthanatrayi in the Vedanta school.
Is the Shanti Parva banal as claimed by some scholars? I have given below an excerpt from Shanti.
Bhishma said: Even the words heard from an ignorant person, if in
themselves they be fraught with sense, come to be regarded as pious
and wise. In days of old, Usanas said unto the Daityas this truth,
which should remove all doubts, that scriptures are no scriptures if
they cannot stand the test of reason.
Mahabharat Shanti Parva Section CXLII translated by Sri K. M. Ganguli
In my opinion the idea that scriptures must also satisfy the test of reason is a sensational statement and proves the scholars wrong who claim that the Shanti parva indulges in banalities. Is the truth value of the Shanti Parva wisdom diminished if scholars who claim that a mortally wounded Bhishma did not speak them are proved right? I would think the answer is a firm no.