I don’t know if R21.5 will work with that kernel. All releases up to and including R21.4 used the same default config file in the kernel. In R21.5 the default kernel config is not the same as that older default (make tegra12_defconfig no longer matches at R21.5). It is possible Grinch will work on that release, but no guarantee…this is the version where config diverges. It wouldn’t hurt to try though.
About the boot loader menu…you must use serial console via that 9-pin D-sub connector to get a menu (u-boot does not have a video driver). Also, make sure labels differ.
If an alternate extlinux.conf entry is used then you can pick the original boot config when the test config fails and rescue that way (to emphasize, the menu requires the serial console). If you flashed and named mmcblk1p1 during flash, or if your extlinux.conf removed the mmcblk0p1 entry and replaced it with mmcblk1p1 for root partition, then you’ll likely need to flash again.