"Once in a lifetime trip with my children" ... is the key phrase here.
In your opinion, which choice would allow you to
freely and spontaneously participate in this experience, and enjoy being with your children, recording the experience, place and things you do together without turning every picture opportunity into a major production?
Personally, it would be a no brainer decision ... I'd take my two M9s. However, if you get an M9, I'd hold onto the M8 as back-up and not sell it until you return ... and I would offer the same advice if you get the Alpa. You do not want to be without a "Once in a lifetime" back-up IMO.
Your "to sensible" DSLR choice is just that, "sensible". With the Leica and the Alpa, you are limited in reach and lower light abilities, which you may or may not wish you had once you are there (depending on what and how you like to shoot). Take a look at the Sony A55 which is really tiny, takes Zeiss/Sony lenses, and is more fun than a barrel of monkeys to play with. Coupled with a Sony 50/1.4, it provides very low light abilities. Add on longer zoom and you have the reach ... if you think you'd use it.
A cheaper route with lot's of flexibility could be your M8 and a Sony NEX5 with an M lens mount adapter, and a long zoom for the NEX. Back-up, low light ability, and reach in a small package. Small may be VERY desirable if you are traveling from one island to another over the three weeks. "Big" becomes a big PITA really fast when moving from one destination to the next.
If I were traveling to this destination alone or with my wife, with the express purpose of making photographs, I'd take my MFD kit ... but if I was there to experience it all with my children, I would not. We all often feel compelled to use our best tools when an opportunity like this arises ... however, enjoying our families comes first IMO.
My 2¢
-Marc
P.S., this doesn't take into account the desire factor ... if you have a burning desire for an ALPA, and think you could master it in time, then no amount of reasoned thinking or logic can quench that desire ... "Abandon hope all ye who enter here."