Resetting NVRam fixed it, though I had to reset it numerous times. Also updated to latest kexts. Thanks!
Edit: spoke too soon. it only fixed it temporarily. issues comes back repeatedly. i have to turn off the laptop for about 10 minutes, then boot with power unplugged and reset NVRAM 3 times, then boot. upon booting, mouse will work but then becomes laggy again during use, particularly when CPU temperature heats up.
Tried many combinations of older OC versions and older kext versions to no avail. Ran the EFI utilities and no issues detected. Linux continues to work perfectly.