Why did Arjuna have to stand behind Shikhandi

Why did Arjuna have to stand behind Shikhandi to fight Bheeshma when Bheeshma himself would put down his arms when facing Shikhandi and Shikhandi himself could have fought him? TV shows like BR Chopra Bharat caused these kind of misconceptions which spread around like wild fire. This was not the only one,BRC Bharat promoted many false interpretations,specially in war episodes. Sikhandi never got into Arjun’s chariot.He was leading from the front with Bhima and Arjuna as his chariot wheel protectors. kRRitvA vyUhaM mahArAja sarvashatrunibarhaNam | shikhaNDI sarvasainyAnAmagra AsIdvishAM pate || 4|| chakrarakShau tatastasya bhImasenadhana~njayau | pRRiShThato draupadeyAshcha saubhadrashchaiva vIryavAn || 5|| Sanjaya said, “And they marched out, O king, having formed an array that was destructive of all foes. And Sikhandin, O monarch, was stationed in the very van of all the troops. And Bhimasena and Dhananjaya became the protectors of his car-wheels Shikhandi was placed ahead of all and Bhima,Arjuna became his chariot wheel protectors. If Shikhandi stationed in Arjun’s own chariot,how could Arjun become the wheel protector of Sikhandi’s chariot? The war narration on tenth day always said…placing Sikhandi ahead of them..It should not be misinterpreted. Answering second part of the question Bhishma did not put down his arms when he faced Shikhandi.He just avoided him and fought with Arjuna .. Sikhandin, O bull of Bharata's race, quickly covered the grandsire with diverse kinds of weapons. Disregarding those shafts, thy sire Devavrata began, with his shafts, to check the angry Arjuna only in that battle. Clearly given ..Bhishma avoided Shikhandi and fought with Arjuna. So contrary to what was assumed and shown in TV shows,Bhishma never put his weapons down, just he did not counter Shikhandi with arrows.Instead he killed Pandava army and fought with Arjun till his fall. Answering Last part of the question Shikhandi could not have fought Bhishma with such vigour as Arjuna did. His arrows did not cause pain to Bhishma. It was Arjuna who shot arrows from a wider distance and struck Bhishma in his vital parts. That caused severe pain to Bhishma. Then Bhishma, deeply and excessively pierced by the wielder of Gandiva with keen-pointed shafts, once more addressed Dussasana with a smile and said, 'These arrows coursing towards me in one continuous line, whose touch resembleth that of heaven's bolt, have been shot by Arjuna. These are not Sikhandin's. Cutting me to the quick, piercing through even my hard coat of mail, and striking me with the force of mushalas, these arrows are not Sikhandin's. Of touch as hard as that of the Brahmana's rod (of chastisement), 1 and of impetus unbearable as that of the thunder-bolt, these arrows are afflicting my vital forces. These are not Sikhandin's. Of the touch of maces and spiked bludgeons, those arrows are destroying my vital forces like messengers of Death commissioned (by the grim king himself). These are not Sikhandin's. Like angry snakes of virulent poison, projecting their tongues out, these are penetrating into my vitals. These are not Sikhandin's--these that cut me to the quick like the cold of winter cutting kine to the quick. Save the heroic wielder of Gandiva, viz., the ape-bannered Jishnu, even all other kings united together cannot cause me pain. By Bhishma’s own admission other than Jishnu all the others united can not cause him any pain. That was why Arjuna had to strike his grandfather much against his wish. He never wanted to be the one though he said in Upaplavya that he will throw down Bhishma. He desisted from fighting with full mettle..He wanted Sikhandi to do the part but Krishna ruled out his decision and declared.. That which hath been destined before, O Partha, must happen. It cannot be otherwise. None save thee, O invincible one, not even the wielder of the thunder-bolt himself, would be capable of fighting with Bhishma, who is like the Destroyer with wide-open mouth. Slay Bhishma, without any anxiety. Finally destiny desired Bhishma’s fall with the contribution of Arjuna and Shikhandi as per the words of Shiva and Brahmā. Bhishma never renounced his weapons. He fulfilled his word given to Duryodhan that he will kill 10000 soldiers daily. He did that on tenth day also. It is an insult to the great warrior of the time ,Bhishma to promote false notions such as he was weaponless,he did not fight and endured the arrows shot at him silently so on and so forth.Epic described his final battle so elaborately..with clarity..But it works if we read the narration without bothering about TV show adoptions.. There was nothing unfair here. Bhishma himself wanted Sikhandi to be there and do his bit because Shiva granted her wish in previous birth. Same way Brahmā the creator destined Arjuna to do the job. Sikhandi was protected by Arjuna and helped in reaching Bhishma. Still Sikhandi’s arrows can not throw Bhishma down. It is not easy to strike vital limbs from such distance accurately. There Arjuna’s prowess worked. Bhishma knew Sikhandi alone can not make it happen. That was why he intentionally asked Arjun to get involved. Arjuna did not want to be known as Bhishma’s killer. The confusion persisted to a great extent. That was why at many places Sikhandi was mentioned as Bhishma ‘‘s killer. But the one who judged the arrows that caused him pain and threw him down, very well knew who fulfilled the destiny and his wish also.