I'd define a new "protocol" for netifd, but you can use gre.sh and the like as a template. You should probably consider it as a package, much as the gre package is created as well -- package/network/config/gre
Other developers might disagree, but especially with a memory-constraiined device, minimal is better.
Kernel patching is done after the sources are downloaded. You can see the patches in the OpenWrt git archive. The kernel patches seem to be under ./target/ and also see https://openwrt.org/docs/guide-developer/patches