  Despite the importance of isotopically purified samples in current
experiments, there have been few corresponding studies of spin qubit
decoherence using full quantum bath calculations. Isotopic purification
eliminates the well-studied nuclear spin baths which usually dominate
decoherence. We model the coherence of electronic spin qubits in silicon near
so called Clock Transitions (CT) where experiments have electronic $T_{2e}$
times of seconds. Despite the apparent simplicity of the residual decoherence
mechanism, this regime is not well understood: the state mixing which underpins
CTs allows also a proliferation of contributions from usually forbidden
channels (direct flip-flops with non-resonant spins); in addition, the
magnitude and effects of the corresponding Overhauser fields and other
detunings is not well quantified. For purely magnetic detunings, we identify a
regime, potentially favourable for quantum computing, where forbidden channels
are completely suppressed but spins in resonant states are fully released from
Overhauser fields and applied magnetic field gradients. We show by a general
argument that the enhancement between this regime and the high field limit is
$< 8$, regardless of density, while enhancements of order 50 are measured
experimentally. We propose that this discrepancy is likely to arise from
strains of exclusively non-magnetic origin, underlining the potential of CTs
for isolating and probing different types of inhomogeneities. We also identify
a set of fields, "Dipolar Refocusing Points" (DRPs), where the Hahn echo fully
refocuses the effect of the dipolar interaction.
