I've found out the following:
/system is read only and cannot be remounted with r/w using the "su" available.
powering with both volume buttons pressed boots into some mode where the system is r/w but any changes made are not kept. I think it is FTM mode, the device is then recognized by ADB but not by FASTBOOT.
powering with vol down button pressed boots into download mode and the device is not recognized by ADB or FASTBOOT.
Any help understanding how to:
1. modify the /system partition
2. what can be done in FTM mode, and why changes to system do not stick
3. how to possibly use download mode to flash modified imgs?
4. find out if any write protection is enabled on the device