Yes, there are 3DS brickers, but all of them brick the NAND only and are recoverable (except when the eMMC controller is locked read-only, but it only works with a specific NAND chip and I'm not aware of anyone knowing how to do it).
As for MCU brickers, there have been unsuccessful attempts, and some have been completely lost from existance without anyone running them, so we don't know if those were functional at all.
It doesn't brick your 3DS. The (questionable) files it uploaded were used to check if someone is using a real 3DS or someone is trying to hack into the system with garbage data. Now that the UnbanMii servers are dead, it's completely harmless.
Absolutely nothing. The servers are dead, so it should just error out and not do anything.
I remember the malware.wikia of UnbanMii conaining some bullshit, so take that with a grain of salt.
That was just a joke to make people not stay on 4.5 with menuhax rxTools. Even though old MCU versions were filled with bugs, running my joke program on it did nothing. Remember, THERE IS NO SUCH THING AS INVALID RGB COLOR!
As for why I said 8.x is the safe point, it's because the mcu sysmodule was updated at that version, even though MCU_FIRM itself hasn't been updated in a year when it came out. So technically the latest MCU_FIRM was released in the 7.x system update.