I don't think that point of light is the shuttlebay, if you look at the rear of the Kelvin in one of the trailer shots (which, unfortunately I don't know how to copy and paste in here) there is what appears to be the shuttle bay at the back of the ship.
Re the bridges, yes the Bridge of a starship is a plug-in module that can indeed be ejected to serve as a lifeboat in an emergency. It's also placed there on the saucer and built as a separate module to enable faster upgrades of bridge systems and interfaces etc. You just unplug the old one, plug in the new and Hey Presto!! New shiny bridge!
Which is also why there is a wide variety of bridge designs between even the same classes of starship. There were even theorised to be mission specific modules - for pure science work etc that could be swapped over for certain types of mission.