You know, I've never attempted to mark an IB Island order "Market" to see what happens so I can't answer that part of your question (though just for kicks I am going to try it to Monday cause I like knowing about the oddities)
When in control of TWS myself, I always route manually (and believe that I get better results that way), but I will say that when using stop flavor orders I would always use BEST routing. I would never want to rely on a stop that is specifically routed -- what if you are away from your computer or your computer goes down and the route that you chose for your stop also has technical difficulties? As most of us know, Island had problems with their matching engine last week and was unavailable system wide. I would sure hate to have a stop blown under those circumstances. BEST will route to an alternative in that situation.
JB