Comment # 4 on bug 1099745 from Franck Bui

(In reply to Richard Brown from comment #0)
> 
> Once the user enters the Emergency shell, any attempt to mount the subvolume
> works perfectly fine - it is not "already mounted" by the time the sysadmin
> can login to the Emergency shell
>
> [...]
> 
> Due to the failure of the boot at this point, systemd drops to the emergency
> shell
> 
> The system is therefore unusable, hence the critical severity of this bug.

>From the emergency shell, you should be able to mount manually the subvolume
and resume the boot proces by exiting from the emergency shell, no ?