Ganguli was right. Irawati Karve in her book Yugānta also states that Śānti-parva is an interpolation and not part of Vyāsa's original Mahābhārata:
Even at the end Bhishma's fate pursued him. He
did not die by Arjuna's arrows. He only fell down
wounded. Now he could have used his father's gift
and found release. But the sun was in the south; dying
souls could find no rest. Bhishma had to use his blessing
to prolong his death for six more months. For six
months Bhishma's body lay immobilized, but his eyes
could see, and with them he had to watch the carnage
of the Kuru clan. He could hear, and with his ears he
had to hear the laments of the widowed Kuru women.
He could talk. And with his lips, later authors made
him speak the banalities of Shantiparva.
Had Bhishma accomplished anything in keeping
his vows? The question remains.
Also, it appears there were originally 4 slightly-different versions of the Mahābhārata:
In this story, too, the Kshatriya hero came out better than Parashurama.
Without complaint Karna accepted the curse, as he had
accepted the training of his teacher. This story does not
deserve much attention. At the time of the cattle raid on
the Viratas, Arjuna had completely routed Karna in an
open battle. It was, therefore, hardly extraordinary that
he should have defeated him again in the last fight. The
object of the story is obviously to show that Karna was
a great warrior and he would not have been defeated
except for the curse of Parashurama. According to
legend, each of the four disciples of Vyasa has given a
slightly different version of the Mahabharata story. The
present version is supposed to have been told by
Vaishampayana. The same story is also said to have
been told by Jaimini. The Kauravas and the Pandavas
quarrelled, they fought a war, the Pandavas won, and
their descendants ruled Hastinapura — these were facts
that Jaimini could not deny. But his version of the story
is said to be partial to the Kauravas. Of this version
only the Ashvamedha chapter is extant. In it he shows
that Arjuna was defeated many times, and each time had to be rescued by Krishna and others. The fact that
Karna was killed by Arjuna was indisputable. The story
of the curse is obviously an invention to avoid the
conclusion that Arjuna was the greater hero. In this
whole episode there is nothing that contributes to the
main story of the Mahabharata.
She further explains how exactly the original Mahābhārata was interpolated:
The late Professor V. S. Sukthankar has pointed out that the Mahabharata saga came into the hands of the Bhrigus, a Brahman clan. These Brahmans inserted the
stories of their own family into the narration of the
Mahabharata. All the Brahman stories referred to above
are part of these later interpolations. They have no
relationship whatsoever with the original story of the
Mahabharata. We can, therefore, dismiss them. If all
these accretions are dropped, the Mahabharata gains in
beauty, economy, and movement.
Did Bhishma really live for another 6 months?
This is what the Ganguli tr. says:
Having slain in that battle, O monarch, (his foes) by hundreds and thousands, there was not in Bhishma's body space of even two fingers' breadth that was not pierced with arrows.
Imagine a 90-year old getting stabbed 100 times in his torso. What are the chances of him surviving for another 6 months with or without medication? Very slim.