Now I see what is really going on. I am trying to reference MCs from my "common.swc" lib which is in a different project, and the reason they are showing up and available is because I used them in MXML files within the "common" project that are called upon and loaded before the mobile specific login screen MXML is loaded.
I just did a quick test to see if the new button I created would be available within the "common" project and it is there, and shows up with intellisense etc etc as expected.
When I add an instance of my new button to a MXML file being loaded before the mobile specific stuff I can then drop it into my mobile specific screens and it shows up.
I either need to figure out how to reference MCs within a swc inside of a different project directly, or just declare them when the application first loads.
All I did was put a reference to the "/Common/libs" path in the Flex Build Path / Library Path tab and now it seems to be working correctly. I hope this is the correct way of doing it.
I have never really used Flex but know AS and the Flash IDE very well, and got tossed into a large Flex application with 600,000 + lines of code and tons of packages etc trying to re-do the User Interface and every single UI element has been a complete struggle to create even though I built mock-ups of the entire application's UI elements in the Flash IDE in a day.
Hope this thread helps someone else who is having this trouble, and going through the same stumbling blocks I have been for the last few weeks.
If anyone else has $.02 to weigh in on this subject please do.