Hi Jason,
The kernel headers are published and available for the versions that are listed in the dashboard for image download. For the Beaglebone Green Wireless the 2.58.3+rev4 headers are currently available only in staging, so you can switch “cloud” to “staging” in https://github.com/balena-os/kernel-module-build/blob/93da0fefec03692c0b5a819355d599b449b9c5c7/build.sh#L5 to get the headers from there.
Yes, all beaglebone use the same kernel, and 2.58.3+rev4 uses the kernel revision 5.4.70-ti-r19, as visible here.
Do you need us to update to r20? Also, if you have some kernel module that you need in the OS and it is public, sure, you are welcome to PR it in the balena-beablebone repository.