Of course. Just like with anything in linux you have many ways to accomplish what you want to do; to get a bit-by-bit accurate backup of your partitions you can use dd, however tar or rsync should be enough to backup the system. Up to a point, yes. If you send your device for repair, then they might set up the baseline of your device to some later OS version. There are lots of patches, but I don't think there are "unofficial versions" of SFOS. Alternative OS'es, yes; you can install Nemomobile/Glacier to it I think. Everything can be modified. ssh client is sufficent