The approach you are taking is not worth it for you simply because:
USB debugging needs to be enabled beforehand for using any adb command on a device. Since your mother probably would have never felt (and why would she anyway?) the need to enable it, assume it to be disabled.
Since debugging can only be enabled either manually by the user under Settings ...
Firstly the main Android OS (what you call "boot mode") and recovery are two independent operating systems. When one is running, other is not. Running processes are different, environment is different, loaded properties are different and so on.
Recovery has its own default.prop file in ramdisk from which it reads properties. In order to permanently change ...
It seems that OP got an answer from Xda-developers for Microwear H5.
You can enter recovery mode via adb command or installing a reboot to recovery app from play store.
The adb command is the usual one for rebooting to recovery:
adb reboot recovery
The answer also applies for Kingwear KW98.
I solved my problem with this tool.
Download the zip;
Unpack the zip in a folder;
Reboot the OnePlus 6 in fastboot mode (Power and volume-);
Connect the OnePlus 6 to PC;
Run flash-all.bat flasher you need;
Wait until the process end;
The phone will automatically reboot.