A previous study of nuclear-spin-lattice relaxation in solid mixtures of orthohydrogen and parahydrogen is extended to include the crystal- and magnetic-field effects which have been observed experimentally at low o-H2 concentrations c. The dynamical rotational angular momentum correlation functions which determine the spin-lattice relaxation rates are calculated as functions of c and crystal- and magnetic-field strength using the self-consistent Sung-Arnold method to treat the electric quadrupole-quadrupole interactions and a rotating-frame transformation to treat the crystal and magnetic fields. Numerical solutions for these correlation functions are then used to obtain the relaxation times T1 and T2 as functions of c, applied-magnetic-field strength, and crystal-field energy. The results obtained for T1 are in excellent agreement with the recent experiments of Buzerak, Chan, and Meyer. On the other hand, there is a major qualitative and quantitative discrepancy between the predictions obtained here for T2 and the experimental results of Buzerak et al. The reason for this discrepancy is not understood. In addition, the dependences of the spectral density functions and nuclear free-induction decay on o-H2 concentration and magnetic- and crystal-field strenghts are calculated.