Stuck at recovery console after upgrade

[Gnom]

Software
XBian version: 1.0RC2
XBMC/Kodi version: XBMC 12.2
Overclock settings: default

Hardware
Device type and model: Raspberry Pi Model B 512 MB
Power supply rating: 1A
SD card size and make/type: Sandisk 4 GB
Network (Ethernet or wireless): Ethernet
Connected devices (TV, USB, network storage, ...): Projector via HDMI to get output, nothing else for test purpose

Log files
None so far

Problem description:
I've been running XBian on a raspberry model B 512 for a couple of years now and I've been keeping it up to date by running apt-get update; apt-get upgrade. Today while researching info about upgrading XBMC/Kodi I learnt that it wasn't the proper way to do it, from the FAQ and other places. I've been trying to upgrade my setup to the current version but I'm encountering problems. So far I've tried :

• Running what's at the top of the FAQ about xbian.brantje.com being unreachable, to make sure that apt gets the right versions of stuff :

Code:

sudo -s
echo "deb mirror://apt.xbian.org/mirror.txt stable main" > /etc/apt/sources.list.d/xbian.list
apt-get update; apt-get install --reinstall lsb-release xbian-package-repo; apt-get update

• Doing what's advised in the "Beta2 [UPGRADE INSTRUCTIONS ONLY!]" topic :
checking what packages are being held back and running

Code:

apt-get install xbian-update list_of_held_back_packages

In both cases, reboot hangs at:

Code:

FATAL: only the last partition can be resized
Mounting root as: mount -t ext4 -o rw, /dev/mmcblk0p2 /rootfs
[ 5.825097] EXT4-fs (mmcblk0p2): VFS: Can't find ext4 filesystem
mount: wrong fs type, bad option, bad superblock on /dev/mmcblk0p2, missing codebage or helper program, or other error
In some cases useful info is found in syslog - try dmesg | tail or so

Mounting root partition /dev/mmcblk0p2 failed

then the XBian invite, then a "Welcome to recovery boot console" explanatory text saying I can chroot int my install and exit from recovery shell at which point my boot would supposedly continue.
I'd gladly try something there but it happens that I can't get my hands on a keyboard. I do have access to the raspberry though so I reverted my card with an image of my system before the update (actually did a few times to try different ways of upgrading). Does anything I could do or check in this situation come to mind?

Here and there I saw some tweeking with cmdline.txt but I didn't see what was relevant to my case, so here's what it looks like now:

Code:

telnet zswap.enabled=1 zswap.compressor=lz4 sdhci-bcm2708.sync_after_dma=0 dwc_otg.lpm_enable=0 console=tty1 root=/dev/mmcblk0p2 rootflags=subvol=root/@,thread_pool=2,autodefrag,compress=lzo,commit=120 rootfstype=btrfs rootwait smsc95xx.turbo_mode=N logo.nologo quiet noswap loglevel=0 mod_scsi.scan=sync partswap startevent=mountall splash nohdparm --startup-event mountall

How to reproduce:
Probably specific to my mistaken usage.