I don't know the answers for sure but some things you can try:
- After disabling SIP, remounting / as read-write so you can copy the new kernel into /System/Library/Kernels with something like "mount -u -w /", which worked for me
- Regarding the second error, it loks like the SDK you're using is different from the OS you're trying to install the extension into. Double checkin your build settings for your kext, and perhaps some verbose output from kextutil would help. Try adding -v 6 to the command line.
Thank you, after diabling SIP and mounting with "sudo mount -uw /" worked. I was able to collect kernel debug core for further analysis and fixed my crash issue.
For second error, I had to use older Xcode 9.4.1 in order to support MAC OS 10.14.x OS.
Here, is the link that I followed,