I am not sure if Calabria is saying that it is illegal to write down the SPS. The CBO GSE Paper envisions several scenarios and handles the requirements for a writedown in Footnote No. 30:
Section 902.2 of title 31 of the Code of Federal Regulations sets forth standards for the “compromise of debts” by the Administration. On the basis of its review of those standards, CBO believes that a reduction in the value of the Treasury’s preferred shares could be undertaken as part of the recapitalization of the GSEs.
The GSE paper states that "The CBO believes that a reduction in value of the Treasury's SPS could be undertaken in compliance with Section 902.2." It doesnt seem that there is a legal requirement but rather the requirement that a compromise of debts follows regulatory requirements which of course is subject to the vagaries of an administrative process.
You imply the redemption value of the seniors is based on their LP. However the CBO report uses face value $190B in their calculation, which remains unchanged between 2023 and 2025. Does Calabria’s book specify which one they were assuming for redemption?
If senior redemption is based on LP then Treasury’s percent of the conversion will keep growing while JPs will shrink. That seems be a disincentive for Treasury to release quickly.