How can I mount the root Data volume as writable on macOS 10.15 Beta 1 when booted into Single User Mode?

I need to reset my admin password (https://apple.stackexchange.com/a/61932/17533) . Can't boot into Recovery Volume, so I need to boot into Single User Mode and modify the Data volume. When booting into Single User Mode, the system shows the following help: To mount the root device as read-write: $ /bin/mount -X / However, that leads to an error message: mount: illegal option -- X I also tried the old mount -uw -, but that leads to some *firmlink* related messages, and afterwards I still have no writeable Data area. mount shows that the Data volume is already mounted at /System/Volumes/Data as read-only. Trying to unmount it with umount leads to an "Operation not permitted" error. mount -uw /dev/disk1s1 (which is the Data partition as shown by mount) leads to the error "Found 2 volumes in group ... Failed to stat file //AppleInternal, error No such file or directory".