lol wow thats a lot of questions.. Umm I use Visual Studio 2010 x64, I have Visual Studio 2008 aswell.
I dont think CodeBlocks for windows is 64 bit. And other than the VS 2010 and 2008, I dont know of any other 64 bit compilers :S
Yes PostMessage & SendMessage will work as Im using it atm to Send Messages from Threads. So I guarantee those functions will work.
I Have also taken a Program I wrote in 32bit vs2010 and compiled it in Vs2010 x64.. There is one problem though, If your importing specific DLL's, Some people say some of them will give trouble. Other than that, *I* have been successful 100% compiling my programs.