Who is 'I' in this verse a specific deity or supreme Bramhan?

Question

I don't know Rishi (my guess is that Rishi's are agniras and Atharvana/Bhrigu) or gods of the following verse of 11th hymn and 3rd kanda of Atharvana Veda

यदि क्षितायुर्यदि वा परेतो यदि मृत्योरन्तिकं नीतएव | तमा हरामि निर्ऋतेरुपस्थदश्पार्शमेनम् शतशारदाय ||

अ०वे० काण्ड-३ सुक्त-११ मन्त्र-२

Be his days ended, be he now departed, be he brought very near to death already, Out of Destruction's lap again I bring him, save him for life to last a hundred autumns.

Atharvana Veda kanda-3 sukta-11 mantra-2

Here the mantra says be 'his', now 'his' is clear from the notion of reader or the one who is astika and follows vedic order.

Since every veda mantra has a god and a rishi , the word 'I' can refer to that god , If no then it must refer to bramhan as he is all powerful and god of gods. So who is the 'I'.

Disclaimer -: Atharva Veda mantras cannot be recited without consultation with a learned man(in vedas)/It should be recited by a learned man. If recited without consultation they produce no or opposite effects.

Answer 1

Verse 2 of Book 3 Hymn 11 of the Atharvana Veda is the same as verse 2 of Book 10 Hymn 161 of the Rig Veda:

1. FOR life I set thee free by this oblation from the unknown decline and from Consumption;
   Or, if the grasping demon have possessed him, free him from her, O Indra, thou and Agni.
2. Be his days ended, be he now departed, be he brought very near to death already,
   Out of Destruction's lap again I bring him, save him for life to last a hundred autumns.
3. With hundred-eyed oblation, hundred-autumned, bringing a hundred lives, have I restored him,
   That Indra for a hundred years may lead him safe to the farther shore of all misfortune.
4. Live, waxing in thy strength, a hundred autumns, live through a hundred springs, a hundred winters.
   Through hundred-lived oblation Indra, Agni, Bṛhaspati, Savitar yield him for a hundred!
5. So have I found and rescued thee thou hast returned with youth renewed.
   Whole in thy members! I have found thy sight and all thy life for thee.

Now as you can see in the Rig Veda Anukramani in my answer here, the sage who heard this hymn from the gods is Yakshmanashana Prajapatya. So that would be the one who is referred to as "I".

Now Yakshmanashana means "curer of Tuberculosis", and indeed this hymn is used to cure Tuberculosis as described in this chapter of the Ashvalayana Grihya Sutras. And Prajapatya means "son of Brahma", but in the Anukramani Shaunaka uses gives the last name "Prajapatya" to pretty much any god who is so old that Shaunaka can't fathom his parentage. For instance he refers to Vishnu as Vishnu Prajapatya, despite the fact that Vishnu has no beginning or end as I discuss here. And he refers to Brahma himself as Hiranyagarbha Prajapatya, i.e. he refers to Brahma as the son of Brahma!

So all we can conclude is that the "I" in the hymn refers to a being who cures Tuberculosis and who is either the son of Brahma or someone just as old.