1) yes ofcourse, 'included' PHP code is inserted into the script at the point where the include() is found, the two scripts become one, and act asif they are one script.
2) it is good if you keep things clear.
It is never ever good if an included scripts only works if the 'parent' scripts set certain vars. The best way to use includes is to write functions and put the functions in the included files.
A forum, a FAQ, email notification, what else do you need?