Hi @je-s,
If the layer is not there then the other questions are not relevant anymore, because that explains your problem fully.
Unfortunately it’s not just the fw_env.config, but actually the u-boot binary itself on the boot partition that you need to update, at least. Not sure if other things might be affected too, but at least from a first glance it might be good enough. Please note that changing that is definitely a single point of failure and might brick the devices. If the devices are easily accessible, rolling out fresh SD cards might be a more time efficient solution, as you don’t need to construct the application update artifacts and test the process.
Greetz,
Josef