Source: TW
The view that iyādi pūrṇaḥ sūtra is just a theoretical construct is also prevalent and hinted at by some texts - citing the taittirīya śruti on ‘sūrya’ “having” three akṣaras (“sūrya iti trīṇi”), the mārkaṇḍeya smṛti forbids the metrical reconstruction of gāyatrī
Relevant ślokas on ṇya “containing” two akṣaras “within” just like the śruti says for sūrya
ण्यमेकम् एव वर्णस्तु
संख्यायां तद्द्वयं भवेत् ॥
णयोःस्वीकृत्य भेदं वै
संख्या सा कथिता परा ।
चतुर्विंशतिका सर्वैः
त्रीणि सूर्ये यथा तथा ।।
And furthermore it says “the japa is to be done as it occurs in veda”
ज्ञात्वा तु वैदिकैः सर्वैर्
जप्यो वेदे यथैव सा ।
गायत्री सर्ववेदानां
जननी ब्रह्मनामिका ।।
This seems to be one of the bases for TN smārtas to not use vareṇiyam - nevertheless there are also pramāṇas cited by those using vareṇiyam in japa (promising disastrous consequences for vareṇyam camp). Personally I think it’s best to follow one’s upadeśa paramparā
The pratiloma gayatri of AV parishiahta presupposes vareniyam only. As you said there are references that condemn both and it’s best to follow what is taught in your tradition.
In some SV tradition, varenyam for arghya and vareniyam for japa.
Yeah the Ahobila mutt and munitraya sampradaya follow vareṇiyam for japa. But between them there seems to be a difference of opinion on whether to use vareṇiyam for prāṇāyāma with M3 preferring to and AM not preferring to.
The issue seems to be that the discussion of chandas is done by hārīta veṅkaṭanātha (tozhappar) only in the prāṇāyāma prakaraṇa. While recommending the chandas as “devī gāyatrī” he doesn’t make an explicit reference to vareṇiyam - I guess this led to diff of opinion.
hārīta veṅkaṭanātha argues that since some smṛtis have named the chandas to invoke for japa of the gāyatrī mantra as “devī gāyatrī” we should just use that as the name of the chandas. He poses this as a response to a pūrvapakṣin asking “but vareṇyam implies nicṛdgāyatrī?”
In his response he doesn’t explicitly clarify what the use of “devī gāyatrī chandas” entails since he also says chandas for praṇava is devī gāyatrī - hence probably the difference between AM and (some) M3 SVs w.r.t prāṇāyāma vs japa