adb and fastboot are different modes. you can enter recovery without usb debugging. Press and hold Power + Vol Down. This is the bootloader screen. Now navigate to recovery with Vol Keys, Power to confirm. Wait a Moment. When the android symbol appears, press Vol Up + Power to enter recovery. Now do Factory reset.
If device is locked you may not be able to enter recovery. you can unlock it from fastboot and try again. Unlocking will factory reset your tablet. Download
platform_tools from google. Enter fastboot mode from bootloader screen. Connect the usb cable. Install the drivers. Open the
cmd.exe as administrator. navigate to the folder. check with fastboot if the serialno appears. unlock.
fastboot oem unlock
then boot TWRP and see if partitions can be mounted. at least /system and /cache should be mountable, otherwise something is wrong. /data is encrypted but should be mountable too. for more info see here. Do not flash TWRP to recovery partition, first backup stock recovery!
fastboot boot twrp-3.3.1-0-shieldtablet.img
in twrp, there is a option reboot recovery which should you bring to recovery mode. if not, you still have to option to backup/wipe partitions from twrp. start with wiping cache. if phone is still not booting, reboot into twrp and format data. if any error occurs, you can pull the log from adb at any time. post the log file in the twrp support thread and ask for help. try another twrp version (3.1.1-0) depending on your android version
adb pull /tmp/recovery.log C:\Users\Admin\Downloads\platform_tools\recovery.log
if you need to enable usb-debugging from recovery, you may modify /system.
(but also the other methods you linked above should work)
/system/build.prop and edit the following lines with Notepad++ (make a copy for backup)
mount /system writeable, then push it back to phone. if you don't want to modify /system you can try to push it
/data/local.prop (but i guess it is not supported on this rom) or do create a file /data/property/persist.sys.usb.config (i don't have exactly read this posts above)
adb push "C:\Users\Admin\Downloads\platform_tools\build.prop" /system/build.prop
Now place the RSA-Key somewhere on the phone and merge it into
adb push "C:\Users\Admin\.android\adbkey.pub" /tmp/adbkey.pub
mkdir -p /data/misc/adb
cat /tmp/adbkey.pub >> /data/misc/adb/adb_keys
if root access is required, you can change
ro.secure=0 in build.prop too (but not recommended)