Can "adb remount" be used on a stock phone (unrooted)? Is it possible to use adb remount with a standard firmware and without having rooted the device? If not, is there another way to force the system to make /system writable without rooting the device?
Without root: No, for security reasons this command is only accessible for the superuser.
Standard firmware, rooted: Yes, if the partition you like to remount is not in a locked flash region (HTC calls that S-on/S-off)