What Each Cosmological Constant Mechanism Leaves Free: An Eight Requirement Audit

Proposed solutions to the cosmological constant problem are developed in largely separate literatures: global constraints and sequestering, unimodular gravity, q-theory, energy diffusion, causal sets, holographic bounds, supersymmetry and extra dimensions, symmetric twin sectors, and dynamical relaxation. The FADE Collaboration review (arXiv:2210.06810) scored nine modified gravity proposals, plus a general relativity and quantum field theory baseline, against a five condition wishlist and asked for further requirements. We extend that exercise to eight requirements and twenty three mechanisms, including work posted in 2026. Of the 184 cells, 176 were scored twice in separate readings of the primary literature, with a quoted equation or passage per cell; disagreements were settled by a third reading. The two readings agreed on 129 of the 176 cells (73%); four cells remain open. Three results stand out. First, no mechanism computes the size of the residual vacuum energy: in every case where a size is given, it rests on an integration constant, a fitted coefficient, a chosen scale, a typicality prior or a selection effect, and the best cases fix only an order of magnitude. Second, no mechanism is shown to remove classical phase transition shifts in all regimes. Third, of the five mechanisms whose leftover is set purely by gravitational scales, four fail on data, on the size of the leftover or on consistency, and the fifth meets those requirements only conditionally or not demonstrably. We also record that, with the stated conventions of arXiv:1901.08588, the residual sourced by pressureless matter is negative, so its self-consistency condition has no positive solution. Version 2 adds a map of 32 routes to a computed cosmological constant that fail, each with the place where the factor 10^-122 ends up, and three results at the current frontier. (i) In Gauss Bonnet sequestering, a sum over topologies generates a potential for the global coefficient of the Euler density; the potential is cancelled but its slope enters the global constraint, so stability needs about 550 e-folds of suppression of Planck scale topology change (wormhole actions above about 10^3). (ii) If the Gauss Bonnet coupling is the sequestered variable, topology generated dark energy and Gauss Bonnet sequestering are mutually exclusive. (iii) In the Kaplan Rajendran mechanism, the stability window is kappa < -1/6, and an operator that, on our reading, lies inside the allowed class restores a cosmological constant with a LambdaCDM form background. We close with a summary of where the problem stands and a graded list of requirements on a quantum gravity resolution. We do not propose a new mechanism. The contribution is a common scorecard and map, with the assumption behind each negative verdict, intended as a guide for further work. Supplementary archive: the version 1 material (two separate readings per mechanism group with quoted evidence for every cell, the adjudicated table, and scripts for the sign check and the alternative-prior calculation), plus a v2 folder with the working notes and scripts behind each map row and frontier result. See v2/README_v2.md for which scripts need public data downloads. Note on versions. Version 1 (10.5281/zenodo.24095412) remains available; nothing in it is withdrawn, and its main table and appendix are unchanged. Version 2 adds Secs. 5 to 8 (map of 32 failed routes, three frontier results, where the problem stands, requirements on a quantum gravity resolution), narrows the motivation for requirement S6 to sequestering type cancellers, and adds the split-problem alternative to the Discussion. No headline number of version 1 changed.