Who else had swallowed the poison then, had Lord Siva not done so? Each and every scripture that has described the Churning of the Ocean episode has confirmed that Lord Siva consumed it. In fact, it was ONLY Lord Siva's ability to swallow the 'Halahala' poison.
And another thing, almost all our authentic scripture including the Vedas, the Upanishadas, the Puranas, and the Mahabharata confirms that Lord Siva can NOT die,
He is imperishable. The Rig Veda has clearly said Him
'bṛhantamṛṣvamajaraṃ' (RigVeda 6.49.10)
Meaning : Great Blissfull and Immortal.
Below are some other such verses :
1.Hridpundariksham viryam vishuddham vicintyamadhye visadam vishokam Achintyambaktyanantarupam Sivam prasanammritam brahmayonim tamadimadhyanthavihinyakam bibhum cidanandarupamabhutam Umasahayam Parameswaram Prabvum Trilocanam nilkantham prashantam dhatya munirgachhyati bhutayonim smastasakshim tamasa parastat || ( Kaivolyopanishad 6-7)
He Who is beyond perception, manifestation, Whose forms are unending, Who is pure, blissful, immortal, the root cause of all the Universe, Who does not have any beginning, middle or end, Who is the only, all-pervading consciousness and beatitude, who is without any form and very much wonderful, that ever magnificent Master Who is conjoint with Mother Uma, omniponent, three-eyed, blue-necked, pacified, the great Yogis attain Him by dint of meditation. He is the source of everything, witness of everything and beyond darkness i.e. Avidya.
nūnaṃ sa deva denānām īśvareśvaram avyayam
jagāma śaraṇaṃ drauṇir ekas tenāvadhīd bahūn |” (MBH 10:17:6)
“Vasudeva said, “Verily, Drona’s son had sought the aid of that highest of all the gods, the eternal Mahadeva. It was for this that he succeeded in slaying, single-handed, so large a number of warriors”.
3.4. “atha kasmāduchyatē anantō yasmāduchchāryamāṇa ēva |
tiryagūrdhvamadhastāchchāsyāntō nopalabhyatē tasmāduchyatē anantaḥ ||” (Atharvasiras Upanishad 4:04)
“Next, why is He (Rudra) called ‘ananta’? Because, his end is available neither at the top nor at bottom, nor at right nor at left – therefore He is called ‘ananta (endless)’”.
4.“sa eva sarvaM yadbhuutaM yachcha bhavyaM sanaatanam.h |
j~naatvaa taM mR^ityumatyeti naanyaH panthaa vimuktaye |”(Kaivalya Upanishad 9)
“He alone is all that was, and all that will be, the Eternal; knowing Him, one transcends death; there is no other way to liberation
5.k ṣ ara ṃ pradhānam am ṛ tāk ṣ ara ṃ hara ḥ k ṣ arātmānāv īśate deva eka ḥ / tasyābhidhyānād yojanāt tattvabhāvād bhūyaś cānte viśvamāyāniv ṛ tti ḥ // Svetatsvara Upanishada //1.10 //
Prakriti is perishable. Hara, the Lord, is immortal and imperishable. The non-dual Supreme Self rules both prakriti and the individual soul. Through constant meditation on Him, by union with Him, by the knowledge of identity with Him, one attains, in the end, cessation of the illusion of phenomena.
6.mahān prabhur vai puru ṣ a ḥ sattvasyai ṣ a pravartaka ḥ / sunirmalām imā ṃ prāptim īśāno jyotir avyaya ḥ // Svetatsvara Upanishada 3.12 //
He, indeed, is the great Purusha, all pervasive and all-powerful. He also inspires the mind to attain the state of purity. He is the Supreme Lord, self-luminous and imperishable.
7.dve ak ṣ are brahmapare tv anante vidyāvidye nihite yatra gū ḍ he / k ṣ ara ṃ tv avidyā hy am ṛ ta ṃ tu vidyā vidyāvidye īśate yas tu so 'nya ḥ //Svetatsvara Upanishada 5.1 //
In the Immutable, infinite Supreme Brahman remain hidden the two: knowledge and ignorance. Ignorance leads to worldliness and knowledge, to Immortality. Brahman, who controls both knowledge and ignorance, is different from both.
Not only this Lord Siva has been stated the Lord of Immortality and knowing Him a person becomes immortal.
puru ṣ a eveda ṃ sarva ṃ yad bhūta ṃ yac ca bhavyam / utāmrtatvasyeśāno yad annenātirohati // Svetatsvara Upanishada 3.15 //
The Purusha alone is all this-what has been and what will be. He is also the Lord of Immortality and of whatever grows by food.
ya eko jālavān īśata īśanībhi ḥ sarvā ṃ l lokān īśata īśanībhi ḥ / ya evaika udbhave sa ṃ bhave ca ya etad vidur am ṛ tās te bhavanti // Svetatsvara Upanishada 3.1 //
The one who spreads the net, who rules alone by His powers, who rules all the worlds by His powers, who is one and the same at the time of creation and dissolution of the worlds - they who know Him become immortal.
These are the verses from the 'Shruti' and a 'Shruti' verse is ever TRUE. Therefore, how could Lord Siva die of consuming the 'Halahala' poison when He is not only immortal but also the Lord of immorality and knowing Him one becomes immortal?
Also, the Taittiriya Aranayaka of the Krihsna Yajurveda confirms Lord Sivais the controller of every created being.:
ishanah sarvavidyanamishvarah sarvabhutanam
brahmadhipatirbrahmano'dhipatirbrahma shivo me astu sadashivom ||
“May the Supreme who is the ruler of all knowledge, ishana, controller of all created beings, the preserver of the Vedas and the one overlord of Hiranyagarbha, be benign to me. That Sadasiva described thus and denoted by Pranava(OM).” (Yajurveda Taittariya Aranyaka 10:21:1 )
So, Lord Siva could easily eliminate the devastating ability of the 'Halahala' poison at His own will ( as the 'Halahala' poison is a created being).
But He swallowed it instead to teach the world the ideals of sacrifice