Results of proton dynamic nuclear polarization (DNP), nuclear spin-lattice relaxation, and ESR experiments in six YES: Er168 (Er168-doped yttrium ethyl sulphate) single crystals at 4.15 K and 9.1 GHz are reported here. This material approximates an inhomogeneously broadened (IHB) ESR line for certain crystal orientations relative to the applied magnetic field H⃗ 0. DNP peak enhancements Epeakss≈120 are obtained for crystals 1-3 having Er168 dilutions of 0.2%-0.01% (mole%). The steady-state enhancement Ess vs H0 curves are proportional to the ESR absorption derivative at low powers (≅ 0.3 mW), but change shape in the wings of the ESR line at high microwave powers (≅ 300 mW). Essentially idential Ess data are obtained for all three crystals. Extensive DNP pump time τDNP and ESR data for crystals 2-4, as well as limited data for the other crystals, are also given. There is no Ess or τDNP evidence in these experiments (i) for the traditional strict-shell-of-influence model of IHB SE (solid effect) DNP, (ii) for non-negligible CE (cross effect) DNP processes, or (iii) for decreases in Ess due to moderate spectral diffusion in an IHB ESR line, as proposed by Buishvili et al. The DNP and ESR data of crystals 1-3 have been fit by the least-squares technique to a muffin-tin model of IHB SE DNP which incorporates rapid nuclear spin diffusion. Moderate agreement with theory is obtained, especially if the data are interpreted with a three-spectral-region model for wide IHB ESR lines. These data have also been analyzed in terms of EDDR (electron dipole-dipole reservoir) DNP theory for an IHB ESR line with fast spectral diffusion throughout the line. There is qualitative and some quantitative agreement between this theory and the data. It is thus difficult to discriminate EDDR DNP theory from IHB SE DNP with the three-spectral-region model in these experiments. The only clearcut evidence for the latter is an order-of-magnitude theoretical calculation of the electron magnetic dipole-dipole contribution to the ESR intrinsic spin-packet width, which gives non-negligible spectral diffusion in the centers of the crystal 1 and 2 ESR lines, and negligible spectral diffusion in the wings of crystals 3 and (perhaps) 2. Conclusive evidence would require further experiments at Er168 dilutions lower than 0.01% and higher than 0.2%. Experiments are proposed for YES: Er168 to test further the three-spectral-region model, and to find high DNP by IHB SE with possible applications to polarized nuclear targets in other, more hydrogenous materials.