You could render from your original comp and tell AE to resize the image to 720×576 with a widescreen pixel aspect ratio. There is no need to letterbox unless you know all screens this will be send to are 4:3 (most likely the vast majority will be 16:9 in which case your piece will look right)
Quick tip, when rendering from AE, no need to render twice if you need various formats – hit the “+” next to your output module and add another output size. AE will render once and produce two files (my mouth was open for a while when I learned about this feature…).
Use the drive that will provide the fastest data transfer, so if the external is faster than your internal drive, then is better to use that, if not, copy all you use on your local drive.
you could also use CC Sphere to explode/implode the roto-ed shape into particles and mix with Particular.
For using Form, look for a tutorial that uses the plugin to replicate the look from a Radiohead music video.
Why don’t you want to precomp? If it’s a matter of having access to the camera, you can always use a Null to control the camera (linked by expressions), duplicate the camera and precomp the text with the duplicated camera (you will have to relink to the Null in main comp).