Phenomenological quantum Hamiltonians H (N ) (λ) = J (N ) + λ V (N ) (λ) representing a general real N 2 −parametric perturbation of an exceptional-point-related unperturbed Jordan-block Hamiltonian J (N ) are considered. Tractable as non-Hermitian (in a preselected, unphysical Hilbert space) as well as, simultaneously, Hermitian (in another, "physical" Hilbert space) these matrices may represent a unitary, closed quantum system if and only if the spectrum is real. At small λ we show that the parameters are then confined to a "stability corridor" S of the λ → 0 access to the extreme dynamical exceptional-point regime. The corridors are N−dependent and narrow: They are formed by a non-empty subset of unitarity-compatible multiscale perturbations such that λ V (N ) j+k,j (λ) = O(λ (k+1)/2 ) at k = 1, 2, . . . , N − 1 and all j.