The /join 0 is a special way of parting all channels, but it's not mIRC doing that, it's really the irc server that is programmed to make you part all channels when it gets that /join 0 command. Now it's almost only used as a (stupid old) joke to new irc users... The comma is used to separate channels, like /join #channel1,#channel2 and is here used to make it less obvious. How many people would not see something fishy in joining a channel without a # in front of it (yes, /join #0 works as expected.)
As for second question: mIRC has no way to know it the last . is the end of a sentence or part of the channel, so it always takes option one. How many channels do you know that end in a . ? Or how many do you find in a /list should you know only strange channels
