Dear Shri Nityananda ji
Firstly let me answer your question with a thought experiment:
If a person from 1900 were to stumble onto a time machine (left there conveniently by someone in the distant future) and travel to Jan 2017 (we will keep the date pre corona for academic purposes :) ) he/she will not be able to understand why people who possess an iphone buy another iphone ...now we have contextual knowledge and hence know that the buyer earlier had version x and is now buying x+1 with extra features and "benefits".
Without contextual knowledge understanding will not come.
You will also have to ask why did Arjuna perform tapasya towards Maheshvara for Brahmashiras when he already had received that astra with all attendant mantras from his Guru Drona. (there are too many references for this in the MB).
He had also previously received all the astras that he later received from the other devatas.
And as you mention the devadutta.
Is this a redundancy?
It is not known in intimate detail how astras operate or what the other attendant "context" of the shankha is, because we as a society have given up most of those contextual roots/rituals and practices.
So what follows falls under the realm of "speculation" as you people would call it.
Indra is the presiding devata of the devadatta shankha and Arjuna himself is an amsha of Indra.
Just as Maheshvara is the presiding deity of the brahmashira /pAshupata Brahma for the brahmAstra and so on.
These astras/ shankha were sentient instruments and not pieces of antique that can be claimed by the likes of Indiana Jones.
The potency of each is directly proportional to the tapasshakti invested in it.
If this were not the case Krishna would not have said: "brahmaNya satyavAdIca tapasvI niyatavrata ..." when describing the prowess of Karna to Arjuna.
Receiving something from the presiding deity is a few orders higher than receiving it from a human Guru.
Think of this as follows: If one approaches a river one can collect so much water (subject to season) if one approaches the ocean one can collect unlimited water (irrespective of season).
Also from the perspective of shakti a disciple cannot in a shaktipata receive more shakti than a Guru is capable of giving. Which makes the Guru and disciple's capacities roughly equal. (Shakti does not operate like water ie I have 10 litres of milk I give 2l to you 2l to X and am left with 6l.....)
Since Arjuna was to fight a battle with his own Gurus, he had to go directly to "source" in order to re-receive.
The effect is apparent in the war of virata where on a single chariot he defeated repeatedly (singly one on one and together when they all attacked him concertedly) all the maharathis of the Kaurava army that included Bhishma, Drona, Krpa, Ashwatthama and Karna...who were acknowledged as the best warriors of their time.
Kind Regards