As long as it fits, you should be good. Sizes M and N are somewhat hard to tell apart (both M and N size plugs will fit in a size N socket, but a M in an N socket will only work intermittently. N won't fit in a M socket though.) But there are any close but not quite there alternatives for the smaller plug size on the battery. So if it fits, it's good.
Yep, just reverse the tip in the socket to reverse polarity. In fact, the cable might have a "+" on one side of the socket and a "-" on the other, and the adaptaplug will be labelled with a "tip". Line up the "tip" to the "-" and you'll have center-negative polarity like the UA5 needs.