I am happy to report that resetting the parameters as ZdroneZ suggested did the trick, and Solo is back in the air nice and stable. Thanks to all who helped me get out of this mess and provided great information.
I diffed out the changes in parameters vs the bad set, and default set (before level and compass calibration). Below are the bad settings that got updated if anyone wants to chime in as to which one in particular would cause this yaw/spin issue.
ACRO_BAL_PITCH , 0.250000
ACRO_BAL_ROLL , 0.250000
ACRO_RP_P , 5.600000
ACRO_YAW_P , 1.800000
AHRS_TRIM_X , 0.018147
AHRS_TRIM_Y , 0.008333
ANGLE_MAX , 2649.000000
ATC_ACCEL_P_MAX , 50400.000000
ATC_ACCEL_R_MAX , 50400.000000
ATC_ACCEL_Y_MAX , 12600.000000
COMPASS_DIA2_X , 0.968792
COMPASS_DIA2_Y , 1.067666
COMPASS_DIA2_Z , 0.969664
COMPASS_DIA_X , 1.056315
COMPASS_DIA_Y , 1.049558
COMPASS_DIA_Z , 0.970788
COMPASS_ODI2_X , -0.049564
COMPASS_ODI2_Y , -0.003396
COMPASS_ODI2_Z , 0.002920
COMPASS_ODI_X , 0.024735
COMPASS_ODI_Y , 0.010715
COMPASS_ODI_Z , -0.042192
COMPASS_OFS2_X , -38.827374
COMPASS_OFS2_Y , 84.543938
COMPASS_OFS2_Z , 86.246162
COMPASS_OFS_X , -85.999916
COMPASS_OFS_Y , 36.340153
COMPASS_OFS_Z , 123.691299
FENCE_ALT_MAX , 120.000000
FENCE_RADIUS , 0.000000
FRAME , 3.000000
GND_ABS_PRESS , 101203.351563
GND_TEMP , 24.269428
INS_ACC2OFFS_X , 0.059225
INS_ACC2OFFS_Y , -0.302268
INS_ACC2OFFS_Z , 1.028521
INS_ACC2SCAL_X , 1.035993
INS_ACC2SCAL_Y , 1.041044
INS_ACC2SCAL_Z , 1.004688
INS_ACC3OFFS_X , 0.093708
INS_ACC3OFFS_Y , -0.122340
INS_ACC3OFFS_Z , -0.818816
INS_ACC3SCAL_X , 0.999938
INS_ACC3SCAL_Y , 0.998889
INS_ACC3SCAL_Z , 0.992297
INS_ACCOFFS_X , 0.007829
INS_ACCOFFS_Y , 0.351049
INS_ACCOFFS_Z , -0.057026
INS_ACCSCAL_X , 1.007704
INS_ACCSCAL_Y , 0.995108
INS_ACCSCAL_Z , 0.989293
INS_GYR2OFFS_X , -0.003961
INS_GYR2OFFS_Y , -0.028506
INS_GYR2OFFS_Z , -0.029795
INS_GYR3OFFS_X , -0.022774
INS_GYR3OFFS_Y , -0.006698
INS_GYR3OFFS_Z , 0.003446
INS_GYROFFS_X , -0.001576
INS_GYROFFS_Y , -0.027051
INS_GYROFFS_Z , -0.011459
MIS_TOTAL , 3.000000
PILOT_ACCEL_Z , 180.000000
PILOT_VELZ_MAX , 240.000000
RC_FEEL_RP , 36.000000
RTL_ALT , 4800.000000
SR0_RAW_SENS , 20.000000
WPNAV_ACCEL , 340.000000
WPNAV_ACCEL_Z , 160.000000
WPNAV_LOIT_JERK , 1800.000000
WPNAV_LOIT_MAXA , 412.000000
WPNAV_LOIT_MINA , 163.000000
WPNAV_LOIT_SPEED , 900.000000
WPNAV_SPEED , 1100.000000
WPNAV_SPEED_DN , 240.000000
WPNAV_SPEED_UP , 320.000000