You shouldn't keep bumping, if people don't answer its because there not interested, don't know or you haven't defined the question well and they couldn't be botherd to ask you to explain it clearly.
I'm also working on a platform game and this is my first time in flash. Firstly, yes, all platforms/walls must be a seperate movieclip. You can however put all platforms/walls inside another movieclip and be able to refer to that parent movieclip, this is useful for scrolling (instead of the player moving, the whole world moves).
As for collison detection, yes you can but all the platform instances in an array if you like but its a little time consuming and everytime you add another platform you have to make changes to the code, don't ask me how to do it (because I can't remember for memory), there is enough information on the web with a simple internet search. However, the better way to do it imo is to make a platform class, and make a movieclip in the libary a member of the class, and just put instances of that movieclip on the stage. If you need to refer to a platform inparticular you can give the movieclip instance an instance name and call it by that.
Doing this is like putting code on an object and using the "this" statement in as2 but better.
Ofcourse, the worst way to do it but is still an option for simple games, you detect and react the platforms individually.
So you have player.hitTestObject(platform001), player.hitTestObject(platform002) etc and create a function for each platform collison reaction. This method involves alot of copy/pasting and changing string names and is the way I used to do it until I learn't about how to use classes in ActionScript 3.0. If your serious about your game and want it done properly, you'll uses classes. Once you learn classes, you'll want everything in a class inculding the npcs or "coins" if its that type of platform game etc.
If you have any furthur related to this problem questions ask them in this thread, if nobody else replies including me, then you can email me at [email protected]
and I'll get back to ya