Nothing special WRT ordering happens when a makefile is included. To make, it's as if you constructed one big makefile where the contents of the included makefile are inserted where the include
statement appears in the parent makefile, then passed the one big makefile to make.
So, the rules for overriding things (variables, etc.) are identical to if you override a variable in a single makefile: each subsequent assignment of the variable replaces the value of the previous assignment.
If you define a (single-colon) rule with a recipe for a target, then do it again later, you get an error message and make ignores one of them.
The first explicit target listed in any makefile, included or not, will be the default target.
Etc.
Remember that make never "executes" any makefile entirely. Make is not a procedural language, like a shell script. It first parses all the makefiles and constructs a dependency graph internally. Then it chooses the default target or targets, and walks the sections of the graph starting with each one.
Edited:
If the above doesn't help you answer your other questions, then I think you should just experiment a little bit. Try it and see what happens! It's easier than asking us to explain it, plus you'll understand it much better if you figure it out yourself.