hello sakata-s,
bootloader logs only available via serial console, you may refer to Jetson Nano Style - Serial Console - JetsonHacks to setup the wiring to gather complete logs.
Q2
you may flash redundant root file systems on an external storage device,
please also refer to Xavier NX’s configuration file, i.e. flash_l4t_t194_spi_sd_p3668_rootfs_ab.xml
Q3
it works, the location of root file system is not related.
the concept of Secureboot is to prevent execution of unauthorized code during boot process through chain-of-trust;
those authenticates boot components (such as, Boot Configuration Table, bootloader binaries, and warmboot vector) were signed using private key.
Q4,Q5
please gather complete logs via serial console for reference.
Q6
please check the developer guide, Software-Based Power Consumption Modeling.
you may access INA3221 power monitor to read the values.