I have a original 3DS and a 3DS xl both on 4.5 but i don't consider the other one a spare, since i mainly used the xl at home and original for travels. Unless you're really that obsessed with the 3ds, I don't see the need for a spare.
You can try unbricking it the same way Rydian mentioned in this thread. http://gbatemp.net/threads/accidentaly-turned-of-dsi-while-updating-r4i-3ds-rts.355172/#post-4783210 Other than that, there isnt much you can do.
This happened to me before, but it haven't occur again since then though. I wouldn't worry too much about it...You can try inserting and removing a game cart a few times to test and see if lock is working properly.
What is the link on your flashcart? You are most likely using the wrong firmware/kernel, since i dont see a R4i SDHC silver on http://www.r4i-sdhc.com/