There is no "alleged" here. Whether someone wants to justify it, or contextualize it or not, the deportations are well documented and no secret. You can find a lot of sources via Marxists.org though expect spin both from the texts and the works they are citing. Neither can adequately cover the whole experience of an ethnic group.
In Blackshirts and Red's Parenti makes a point of saying that these internal deportations are crimes of state and wrong, but that the numbers are often inflated. The mistreatment of the Ingush and Chechens went back a decade prior complicating things.
https://www.marxists.org/history/erol/periodicals/marxist-leninist-research-bureau/mlrb-07.pdf https://www.marxists.org/history/etol/newspape/isj2/2000/isj2-086/ferguson.htm https://www.marxists.org/archive/bland/1993/07/enforced-resettlements.pdf
The 1/4th death toll I think is correct, but that's not for the Mountain SSR deportations alone, that's for the batch of them post WW2.