If only ONE drive fails you have a degraded array - in other words : You have simply MORE drives in the array so the possibility of a double-failure is much higher !
And Linux-SoftRAID-expansion ? There is one tool that seems to work, but this is very very experimental and time-consuming. You simply need a full backup i think.
Uhm... and EVMS ? Doesn't this monster need a bunch of nasty kernel-patches ? You even dont need this GUI (is it much more?), mdadm can do the job too.
Compiling a custom kernel is no big deal, especially on Debian it is quite easy :
Install "krenel-package" and "build-essential", unpack the sources, configure it and type "make-kpkg kernel_image" and you get fine deb-Packets ;-)
First take a look there :