You're right, I missed the initial part about manual upgrade, so after following the manual installation steps both the kernel and alt_kernel partition should contain the same OpenWRT build, so two different openwrtboot versions isn't necessary.
However, what else did I miss? I see that bootusb in commit description assumes image was written to a flash drive by dd, but reading the image from FAT partition should work on Linksys LN1301 too? Or IPQ8072A and IPQ8174 are different in this regard?
Also instead of two new openwrtusb and openwrtboot variables (as proposed by hnyman) in the commit description there is slightly different approach but both should work, right? Or there is something wrong with defining openwrtusb and openwrtboot variables and then running them from bootcmd?