Hello ST3VO, I am a little confused here while I read through all the posts.
I need to know what is your current problem and which problems you have fixed.
Please show the updated code.
If you can attach the source file, it will be great.
This is why I don't understand your question, given I don't read through all the other posts.
Here's what I infer,
your
youtubevideo_mc.loadMovie("http://www.youtube.com/v/Rd-ZXOJDBsg&hl");
does not work because you either have no variable referenced as youtubevideo_mc or you have no instances named as youtubevideo_mc.
this usually refers to _root or the movie clip you are placing the code in. Assuming it's _root, you will get problems because you are overwriting the root with something else, thus removing all your other code.
If it's a movie clip, there shouldn't be any problems.
A restate of the question will be great. More details will be nice, like the instances you have on stage as well as any other code that is linked to the code you are using.
For example if youtubevideo_mc is actually a variable, it will be helpful if you show me the definition.