Thank you, Prof. Ryan, Prof. Deshpande, and Prof. Cardona for your responses. I had been travelling and so could not respond earlier.
Here are some quick comments.
1) ‘śuá¹£rÅ«á¹£amÄṇÄnÄm’ is certainly a digitization error. The reading in BhÄgavata PurÄṇa 1.1.13 is ‘śuÅ›rÅ«á¹£amÄṇÄnÄm’ with the palatal  ‘ś’. The Ätmanepada is from ‘jñÄÅ›rusmá¹›dṛśÄṃ sanaḥ’ (A 1.3.57).
2) The RV example of ‘ajuá¹£ran’ is a non-Paninian form, the Paninian form being ‘ajuá¹£anta’ from the root ‘juṣī prÄ«tisevanayoḥ’ (DP 1288). The two non-Paninian features are the ‘ruá¹â€™ augment and the use of ‘parasmaipada’. SÄyaṇÄcÄrya explains the ‘parasmaipada’ by ‘vyatyaya’ (which would refer to ‘vyatyayo bahulam’, A 3.1.85), and the ‘ruá¹â€™ by ‘bahulaṃ chandasi’ (which would specifically be the rule 7.1.8). I have attached the snaps of the mantra and the commentary (Vaidic Samshodhan Mandal, 1935, Volume I).
3) I have not had the opportunity to cross-check the cited examples against published editions the Åšatapatha BrÄhmaṇa, hence I cannot comment on them now.
4) In a word boundary, a final ‘ṣ’ cannot occur in the Paninian system due to ‘jhalÄṃ jaÅ›o'nte’ (A 8.2.39) which will change a terminal ‘ṣ’ to a ‘á¸â€™
Most likely, the examples of ‘ṣr’ do not occur in the Paninian system.
My interest to trace an attested form was partly because I wanted to see how the conjunct was printed in old books. It appears that the printing press where the Vaidic Samshodhan Mandal book was printed did not have a printing block for the glyph ‘ṣr’ (quite possible given the conjunct is so rarely attested). The ‘r’ hook was put below the glyph for ‘ṣ’, which is unexpected: given the similarities between the glyphs for ‘p’ and  ‘ṣ’, one would expect the glyph for ‘ṣr’ to be similar to that of ‘pr’, with a slanting diagonal line as seen in modern fonts. Perhaps manuscripts of RV would need to be seen to ascertain how the glyph was actually written by scribes before the use of printing presses in India.
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Thanks, Nityanand