Version 2 (22 February 2026) – Substantially expanded full paper (27 pages). We derive a covariant cosmological framework from the single axiom of conserved total informational energy with causal-horizon cutoffs on vacuum modes (Quantised Inertia). In its ultimate first-principles form the framework aims for no free parameters: the auxiliary horizon field ϕ(x) = 2c²/Θlocal(x) is purely geometric (fixed by the expansion scalar of fundamental observers), the thermodynamic coefficient γ is fixed by Brodie’s (2026) Rindler-cosmic horizon overlap integral, and the matter density is envisioned as a statistical relic of early-universe horizon quantization. We present the complete action principle, background dynamics, and linear perturbation equations (scalar + vector sectors, including the novel action-derived vorticity source). A β-free vorticity-coupled modal solver yields positive vorticity growth (exponent +1.9) and an older universe (~32–34 Gyr). Runs of our implemented CLASS-HQIV fork recover the first five acoustic peak ratios ℓn/ℓ1 to within ∼4–18 % of Planck Collaboration (2020), with a systematic shift to lower ℓ attributable to the modified post-recombination expansion history. These preliminary results, together with a unified resolution of the apparent age, acoustic-peak, and σ8 tensions via the same covariant mechanisms, motivate full Planck likelihood optimisation and the planned early-universe horizon-mode simulation. Key additions in v2: • Full covariant action principle and variational derivations (Appendix A) • Explicit linear perturbation hierarchy with geometric ϕ(x) coupling • β-free modal solver + complete CLASS-HQIV fork results (peak ratios, σ8, age) • Detailed responses to Renda (2019) and other critiques • Unified tension-resolution mechanism All code (CLASS fork, modal solver) is open-source at github.com/disregardfiat/hqiv. This supersedes the earlier short version (doi:10.5281/zenodo.18717656). Keywords: Quantised Inertia, MOND, Hubble tension, cosmic age problem, modified gravity, emergent gravity, horizon thermodynamics