The already mainlined kernel patches wouldn't need to be rewritten since they're already in the mainline kernel. I didn't see where you specified that before.
But not all HWE is in the kernel. For example, your userspace VoLTE daemon is extremely useful for those who need VoLTE, but would not be able to be packaged in pmOS so other devices can benefit from it, since pmOS is the de facto mainline Linux distro for mobile (there are others, but pmOS is the largest and where most development is done). Same goes for your fingerprint daemon.
There is, of course, also the ethical matter of using AI, as the pmOS page mentions, but balancing issues of pragmatism and principles have been a challenge in the FOSS community since the beginning (ex: "free software" vs "open source").
On the last point about understanding the code, I feel you've shown that you are able to understand the code that you commit, or at least I hope that you do. I'm willing to take you at your word when it comes to that.