to answer your question, yes, it is possible to combine things into one page, but I will say this: The code requires a lot of cleanup and storyboarding for you to do it.
You have a few options...
1) Use If/ELSEIF/ELSE or CASE/SWITCH statements to determine what part of the validation code to execute.
2) Write everything into functions, and call the particular function you need, based on #1 above.
PHP Freaks has a decent tutorial on how to write a membership system. Maybe begin there to see what they're doing.
I just don't have the time to sit down with all of your code, and tell you how to do it. Sorry. Maybe, once you get to the point of getting it all combined, you should come back to post for help, but until then, you may be hard pressed to find someone with the time to do it for you. My tips above should be helpful to you though, depending on your level of expertise.
