Thermal mass is much more complicated than thermal resistance since unsteady conduction involves propagation. This means that spatial discretization can't be avoided. A reduced-order system can be obtained though by modal analysis and truncation; I.e. projecting down to the slowest few eigenmodes, which is effectively.replacing the finite element discretization with a global spectral one. The mechanics of that, once the generalized algebraic eigenvalue problem on the conduction and mass matrices has been solved (use ARPACK or similar to get just the first few modes) is very like the statical condensation that you've already hit on.
For your first question, I'd say that actually thermal resistance R is only a scalar by shorthand, really its reciprocal, thermal conductance, is a 2×2 matrix of a special form: [[1, -1], [-1, 1]]/ R. This matrix acts on the 2-vector of plate temperatures and gives the 2-vector of inward heat transfer rates. 724
This matrix in this form can be derived by statical condensation using two 'supernodes', one for each plate. This way of thinking about resistance becomes essential if there are more than two plates. The coefficients of the multiplate conductance matrix can be computed from solutions of the steady heat equation either from an integral of the heat flux over each plate or from a volume integral (which is kinetic energy in the hydrodynamic analogue or dissipated power in the electrical). I wrote a worked example in Python, see https://kinnala.github.io/scikit-fem-docs/examples/ex13.html, and a longer explanation in 'Three ways to compute multiport inertance' (2019) The ANZIAM Journal, 60, C140–C155, https://doi.org/10.21914/anziamj.v60i0.14058 .
Going back to the transient problem and its reduced-basis approximation, if you do want a scalar representation, just use a single eigenmode. If you include more than one mode, each mode has its own thermal capacitance and thermal resistance, the product being the timescale of the mode. As a circuit, a modal model has a parallel branch between the plates for each mode and each modal branch has a resistance and a shunt capacitance (capacitance to ground). That the modal branches are in parallel corresponds to the simultaneous diagonalization of the mass and conduction matrices in the generalized algebraic eigenproblem. 724