@eselarm Good question, not yet sure what to do.
First off, new stock Raspbian worked (for me) out of the box, at least for both V4L2 and libcamera. I'd have yelled at them if they didn't, as that's Raspberry hardware and software up and down the stack, so they have no excuses. I mostly want to use libcamera, but for me, libcamera and V4L2 were in total agreement on both Raspbian and Armbian. On Raspbian, both detected the IMX708, and on Armbian, neither did -- or, rather, /dev/video0 never got set up, despite the module loading fine and dmesg logging it in Armbian.
To add, also, with the zram swap system in place, I can't actually compile libcamera (or the apps) on the Armbian Zero 2W, due to lack of memory (!). I ran out the will to work around that at the time. So I built the Debian libcamera packages (all of them) on a Pi 5 and copied them over. Only the trixie-backports modules actually ran through okay, but they did. That seemed to work fine -- and as I said, neither V4L2 nor libcamera set up an accessible camera device -- and they agree -- so I think the issue is kernel-side, not in userspace. I suspect it's in the device tree somewhere. My guess is if we get V4L2 happy, then libcamera will be. The main advantage of V4L2 is that the device list is a clear tell for camera detection on boot, one that doesn't need a much installation work.
Also, I didn't yet get to the rpicam apps framework. I will, because I actually use it for the cow I want to deploy. But that's an extra layer beyond libcamera itself. the standard libcamera-tools package contains the cam binary which is enough to detect devices. If I can get V4L2 happy, or if cam can capture an image, then I know the kernel side is fine, and it's all my problem. I'll be happy to take it from that point.
Your point about raspi-firmware is very helpful. That would fit some of what I've seen here. I'll dig in and check on that. I would not assume it's using a Raspberry vendor kernel. I didn't try that. And Broadcom shenanigans apart, I hope I don't need to. What's weird is the IMX708 kernel module is definitely loaded and active and detecting the hardware on the Armbian Debian trixie. Most of the Broadcom stuff is there -- all the hardware encoders show up. Just not the camera itself.
Many thanks -- I'll dig in today and see if I can progress.