We have six players too. It's tougher (and only one of us has a baby on the way) than it was, but we've managed to make the time so far.
We play once a month too. The solution around "what the hell happened last month" has been resolved by me being a sort of co-DM, filling in the blanks for what our characters do between episodes and writing it all out as a Story Hour. I send it to everyone involved before posting it on ENWorld and ensure they vet what happened (sometimes, we tweak an event so it makes a better story too).
We thus have a log of sorts to always refer to. I bring a book of everyone's characters and the story so far to every session. It's worked really well, but it does require a DM and a co-DM of sorts.
I've thought about dropping players down to 4 and trying to be more consistent about playing, but these guys are my friends and I don't want to cut out two friends in order to play more D&D. So, we'll continue on like we are. It's still fun to get together and play.
The way I look at it? At minimum we need four people minimum (3 players, 1 DM). Three of us are always available (me, my wife, one of my buddies who lives near me) so we will ALWAYS have an opportunity to game. If someone can't make it, we have three other people to take up the slack, which happened last game. Now our problem is we have too many people wanting to play, which we really can't handle within four hours (as it is, six players is a lot).
Of course, the four hour games means that there's never a character disappearing in the middle of an adventure. The game has a definite beginning and ending point, so it's easier to explain if someone can't make it.