  Conventionally for observable $n-{\bar n}$ oscillation through Pati-Salam
intermediate gauge symmetry in $SO(10)$, the canonical seesaw mechanism is also
constrained by $M_R \sim M_C \le 10^6$ GeV which yields light neutrino masses
much larger than the neutrino oscillation data. Recently, this difficulty has
been evaded via inverse seesaw mechanism, but with proton lifetime far beyond
the experimentally accessible limits. In the present work, adopting the view
that we may have only a TeV scale $Z^{\prime}$ gauge boson, we show how a class
of non-SUSY $SO(10)$ models allow experimentally verifiable proton lifetime and
the new contributions to neutrinoless double beta decay in the $W_L-W_L$
channel, lepton flavor violating branching ratios, observable $n-{\bar n}$
oscillation, and lepto-quark gauge boson mediated rare kaon decays. The
occurrence of Pati-Salam gauge symmetry with unbroken D-parity and two gauge
couplings at the highest intermediate scale guarantees precision unification in
such models. This symmetry also ensures vanishing GUT threshold uncertainy on
$\sin^2\theta_W$ or on the highest intermediate scale. Although the proton
lifetime prediction is brought closer to the ongoing search limits with GUT
threshold effects in the minimal model, no such effects are needed in a
non-minimal model. We derive a new analytic expression for the $0\nu\beta\beta$
decay half-life and show how the existing experimental limits impose the lower
bound on the lightest of the three heavy sterile neutrino masses, $M_{S_1}\ge
14\pm 4$ GeV. We also derive a new lower bound on the lepto-quark gauge boson
mass mediating rare kaon decay, $M_{\rm lepto} \ge (1.53{\pm 0.06})\times 10^6$
GeV. The $n-{\bar n}$ mixing times are predicted in the range$\tau_{n-{\bar
n}}\simeq 10^8-10^{13}$ sec.
