Nandroid backup in recovery? MIUI Backup? There's a lot of tools to make a backup before wipe.
This error:
Code:
assert failed: getprop("ro.product.device") == "GT-I9100" || getprop("ro.product.device") == "GT-I9100" || getprop("ro.product.device") == "galaxys2" || getprop("ro.product.device") == "galaxys2" ||
E:Error in /sdcard/MIUI/miuiandroid_GT-I9100-2.5.11.zip
means that you either do not have the right device to flash or.. after you wiped /system you don't have build.prop in /system so the getprop function fails. It tries to read the file which doesn't exist.
If so you can try disabling assert scripting in recovery (just search for this function) or... you can open miuiandroid_GT-I9100-2.5.11.zip (but without unpacking!!) and open file /META-INF/com/google/android/updater-script in text editor (Notepad++) and delete first 2 lines:
Code:
assert(getprop("ro.product.device") == "GT-I9100" ||
getprop("ro.build.product") == "GT-I9100" || getprop("ro.product.device") == "galaxys2" || getprop("ro.build.product") == "galaxys2");
Save and put file back to zip. This should allow you to install zip.