I don’t think the eMMC model has software to boot from SD card. What you’ll probably end up having to do is put everything except the rootfs on eMMC, and then direct rootfs to mount the SD card rootfs. The non-eMMC dev kit model has QSPI memory which does not exist on the eMMC model, and that content is for booting an SD card. The question is whether you really need everything to boot from SD card, or if you are ok with just the rootfs being on the SD card? The former is probably not possible, and the latter is probably (relatively speaking) simple (though I suppose it is possible it might involve an initrd, but also might not).
I can’t help with the detect part, but if the SD card shows up, then mounting partitions will be no different on a Jetson versus any other Linux system. You would probably want to mention a specific use-case of how you want to use the SD card mount to correctly answer the mount question. I say this because auto mount might already work as you want it, but if you want this to be mounted in a specific place, then that would be another step. If you want this to not interfere with boot when it is missing, then that too needs to be known (due to different mount options).