Cannot install grub, segmentation fault, unable to identify filesystem, superfluous RAID member, found two disks with same index — Debian 7

You should answer your own question and mark it as such, as it seems you're going towards a solution. A few suggestions:

• Don't try to install grub on the mdadm device, that won't work, but the actual physical device, i.e. /dev/sdn, it looks like you found that out later on
• First let the array happily rebuild after booting with a CD into rescue mode and only after it has finished rebuilding and is reported to be ok try to do more rescue the system
• When you add a new disk to boot from it, at the time of installing grub on it it may be a good idea to run grub with the --recheck option so it will scan for the other devices and add them, to be found at boot time.

A while ago I wrote something about setting up a mdadm raid10 which allows booting from each disk in the raid, it may provide some helpful pointers:

How to create a bootable redundant Debian system with a 3 or 4 (or more) disk software raid10?