I forgot to mention, that it is something I can't really know, (if 3.9 or 4.6), because this iPhone was given to me about a month ago, (not a 3G), when my uncle went to change his phone for a 3G.
2) In this day and age, there's no reason why it matters. Historically bootloader version determined unlockability, but the only unlocking tool you would want to use now, BootNeuter (through PWNAGE), will tell you your bootloader, let you easily unlock both bootloaders, and let you change your bootloader (if you really want to)!