It's a well-known fact about the Gayatri Parivar's stance on allowing everyone (including married women, non-Hindus, and Śhūdras, etc.) to undergo the sacred thread ceremony (Upanayana or Yagnopavita saṃskāra), thus allowing everyone in the society to become a dwija.
However, as per major/authoritative Smṛtis, Puraṇas and Dharma śhāstras - only the top three varṇa are authorized to be "dwija". Further, for women, only Brahmavadinis may be initiated and not the married (Sadyovadhu). However, they initiate married women too. Further, Śhūdras are forbidden too.
However, here we have the Gayatri Parivar, giving the "dwija-initiation" to all members of the society.
Furthermore, from their "guidebooks" - it seems that they follow the "karma and guṇa based varṇāhśrama vyavasthā". However, as per several QnA discussions, on this site, majority view might be tending to support the Birth-based varṇāhśrama system.
If purely looked at from scriptural perspectives,
isn't this a violation of the scriptures and thus a grave sin?
Is the practice by Gayatri Parivar of initiating everyone with Janaeu an adharmika act?
Or, since they follow the "karma and guṇa based varṇāhśrama vyavasthā" they are exempted from the adharmika tags and thus, no sins committed?
If it's sinful, then what scriptural repercussions will they face after death?
Please give a proper scripturally sourced answer.
NOTE: IMO, this is not an Opinion-based question because I'm asking if their acts or practices are in line or out of line, with the "major" Vedic and Dharmika śhāstras injunctions.