Suffering is more than just mental, I am sure you know that.
Buddha suffered and died for a fact i.e. his suffering did not end.
As to whether he could indeed have the ability to freely extend his
life, this point is now moot since he died. I would have think his
ability to overcome the effects of eating contaminated food or
lived for a few decades or so would go a long way in perpetuating
the truth of his teachings or at least demonstrate that suffering
cannot hurt him.
Buddha may have felt physical pain, but not mental
suffering*.
Three months before his passing, he already predicted and announced
his passing. Means: he had chosen by his own will to pass away.
Why? Because he has completed his job. It's up to us now.
In the Mahaparinibbana sutta it is clearly recorded:
http://www.accesstoinsight.org/tipitaka/dn/dn.16.1-6.vaji.html
7.
And when the Venerable
Ananda had gone away, Mara, the Evil One, approached the Blessed
One. And standing at one side he spoke to the Blessed One, saying:
"Now, O Lord, let the Blessed One come to his final passing away;
let the Happy One utterly pass away! The time has come for the
Parinibbana of the Lord.
"For the Blessed One, O Lord, spoke these words to me:
'I shall not come to my final passing away, Evil One, until
my bhikkhus and bhikkhunis, laymen and laywomen, have come to be
true disciples — wise, well disciplined, apt and learned,
preservers of the Dhamma, living according to the Dhamma, abiding
by the appropriate conduct, and having learned the Master's word,
are able to expound it, preach it, proclaim it, establish it,
reveal it, explain it in detail, and make it clear; until, when
adverse opinions arise, they shall be able to refute them
thoroughly and well, and to preach this convincing and liberating
Dhamma.' [23]
8. "And
now, O Lord, bhikkhus and bhikkhunis, laymen and laywomen, have
become the Blessed One's disciples in just this way. So, O Lord,
let the Blessed One come to his final passing away! The time has
come for the Parinibbana of the Lord.
"For the Blessed One, O Lord, spoke these words to me: 'I
shall not come to my final passing away, Evil One, until this holy
life taught by me has become successful, prosperous, far-renowned,
popular, and widespread, until it is well proclaimed among gods and
men.' And this too has come to pass in just this way. So, O Lord,
let the Blessed One come to his final passing away, let the Happy
One utterly pass away! The time has come for the Parinibbana of the
Lord."
9. When
this was said, the Blessed One spoke to Mara, the Evil One, saying:
"Do not trouble yourself, Evil One. Before long the Parinibbana of
the Tathagata will come about. Three months hence the Tathagata
will utterly pass away."
The Buddha had enormous powers and that is why he could choose
things like that. But powers does not equate to liberation or
awakening, nor does awakening necessarily equate to having powers.
Non-Buddhists also have powers. Those who mastered jhanas and
samadhi can have powers.
Among the Buddha's powers are:
1. The knowledge or ability of attaining insight (With this
knowledge, He knows, “my body is material, made from four great
elements, born of mother and father, fed on rice and gruel,
impermanent, liable to be injured and abraded, broken and
destroyed, and this is my consciousness which is bound to it and
dependent on it.”),
2. the knowledge of the production of a mind-made body, or the
supernormal power of the mind-made body (With this knowledge, out
of this body He produces another body, having a form, mind-made,
complete in all its limbs and faculities.),
3. the various supernormal powers (With the super-normal powers,
being one, He becomes many, and being many, He becomes one; He
appears and disappears; He passes through fences, walls, and
mountains unhindered as if through air; He sinks into the ground
and emerges from it as if it were water; He walks on the
water
without breaking the surface as if on land; He flies cross-legged
through the air like a bird with wings; He even touches and strokes
with His hand the sun and moon, mighty and powerful as they are,
and He travels in the body as far as the Brahma world.),
4. the divine ear (With the divine ear, He hears sounds both divine
and human, whether far or near.),
5. the knowledge of others’ minds [understanding the ways of
others’ thought] (With this knowledge, He knows and distinguishes
with His mind the minds of other beings.),
6. the knowledge of previous existences [the remembrance of one’s
former states of existence] (With this knowledge, He remembers many
previous existences: one birth, two births,. . . . a hundred
thusand births etc.),
7. the divine eye or the knowledge of the passing-away and arising
of beings (With this divine eye, He sees beings passing-away and
arising, inferior and superior, well-favored and ill-favored, to
happy and unhappy destinations as kamma directs them.), and
8. the knowledge of eradicating defilements (With mind
concentrated, purified and cleansed, unblemished, free from
impurities, malleable, workable, established and having gained
imperturability, He applies and directs his mind to the knowledge
of eradicating defilements or destruction of corruptions. With this
knowledge, He
knows as it really is: “This is suffering, the origing of
suffering, the cessation of suffering, and the path leading to the
cessation of suffering.)
(Dighanikaya, Vol.1,Samathaphala Sutta)
* The Buddha taught that this is how liberated beings experience
pain:
http://www.accesstoinsight.org/tipitaka/sn/sn36/sn36.006.nypo.html
..."Having been touched by that painful feeling, he does not
resist (and resent) it. Hence, in him no underlying tendency of
resistance against that painful feeling comes to underlie (his
mind). Under the impact of that painful feeling he does not proceed
to enjoy sensual happiness. And why not? As a well-taught noble
disciple he knows of an escape from painful feelings other than by
enjoying sensual happiness. Then in him who does not proceed to
enjoy sensual happiness, no underlying tendency to lust for
pleasant feelings comes to underlie (his mind). He knows, according
to facts, the arising and ending of those feelings, and the
gratification, the danger and the escape connected with these
feelings. In him who knows thus, no underlying tendency to
ignorance as to neutral feelings comes to underlie (his mind). When
he experiences a pleasant feeling, a painful feeling or a neutral
feeling, he feels it as one who is not fettered by it. Such a one,
O monks, is called a well-taught noble disciple who is not fettered
by birth, by old age, by death, by sorrow, lamentation, pain, grief
and despair. He is not fettered to suffering, this I declare.
"This, O monks, is the distinction, the diversity, the
difference that exists between a well-taught noble disciple and an
untaught worldling."