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/
It doesn't show there's an update because you probably have the older version of that cart that doesn't support newer firmware. These R4 clones company usually make new version of the cart, forcing people to buy their new cart/another cart for updates to work.
I experienced the same problem removing this crappy screen protector off a DS lite, what i did is, I used a hair blow dryer on warm setting for about 20-30sec? After that it came off easily without much force.
Hopefully the "converter" you're talking about is actually a step up transformer, not just any normal converter. But it's probably too late, the moment you plugged the n64 into the wall socket without a transformer, somewhere in the n64 is damaged.
Sandisk 2 GB
Sandisk 16 GB class 10
Transcend 8 GB class 6
Patriot 16 GB class 4
Kingston 8 GB class 4
Kingston 16 GB class 10
All of these works fine for me so far...
You pretty much ran a firmware update MADE for another cart on yours and bricked it, in order to fix it you will need another working flashcart, and a DS phat or DS lite.