  We present a demonstration of remnant structural universality in a putative
S-wave 2n-halo-bound $D^0nn$ system in the J=0, T=3/2 channel in an idealized
zero coupling limit (ZCL) scenario eliminating sub-threshold decay channels. In
particular, we estimate the one- and two-body matter density form factors along
with the associated mean square distances and the $n-D^0-n$ opening angle. For
this purpose, we carry out a leading order investigation in the context of an
effective quantum mechanical Faddeev approach in the momentum representation.
Using Jacobi momentum representation we build a complete set of partial wave
projected basis in order to reconstruct the full three-body $D^0nn$
wavefunctions in terms of distinct subsystem rearrangement channels. By
projecting onto the chosen basis, a set of coupled Faddeev integral equations
are derived that describe the re-scattering dynamics of the coupled spin and
isospin subsystems. Furthermore, by introducing short-distance two-body
separable interactions, along with the scattering amplitudes (T-matrices)
expressed via the so-called spectator functions, we establish a one-to-one
correspondence with the well-known Skornyakov-Ter-Martirosyan integral
equations used in the context of pionless/halo-EFT at leading order. A study of
the regulator dependence of the integral equations reveals the underlying
Efimovian character of three-body observables; in particular, those of the
ground state being quite sensitive to the scale variations. Nevertheless, with
the integral equations and three-body wavefunctions modified by the three-body
force, such regulator dependence gets asymptotically suppressed in agreement
with the renormalization group invariance. It is thereby concluded that for
sufficiently small ground state three-body binding energy, the $D^0nn$ system
in the ZCL scenario assumes a universal halo-bound structure.
