Well, first off your basic question:
When you open a Raw file in the Photoshop editor, what you have is no longer the Raw, but is a "rendered" image, and, unless you open it as a Smart Object, your Raw data is no longer a part of the image. Raw files opened as a Smart Object can be re-opened in a Raw processor, but that is not something that will apply to a Panorama once it has been stitched.
Once you have applied editing to the image (including stitching) and want to save it, you can't save it as a Raw file but only as an image file, meaning typically a psd, a tiff, or a jpeg.
So, as to your Panorama, the best quality files for any further procesing are tiffs and psds.
Now as to dealing with Lightroom, with the stitched panorama, you have two choices:
A straightforward choice from the perspective of a Lightroom workflow is to select the Raw files you want stitched and use the Edit In/Merge to Panorama in Photoshop. Then in PS you do the merging, then do a file/save as a tiff or psd, and the saved panorama should automatically show up in your Lightroom library where you can do whatever further development you want.
The other way would be to open your files in Photoshop, either as individual files from Lightroom or through Bridge using the Photoshop/Photomerge function, create and save the panorama, and then in Lightroom you can import it either using Import or the Folder Synchronize function.
Like I said, using Lightroom is the "straightforward" approach.
As to your question of processing the Raw files in Lightroom before doing the panorama work, well I guess I'd say "it depends"...depends on the specifics of the image and on how confident you are that you can make specific adjustments without screwing up the Photo Merge!
A couple important things to check on before the stitching would be ensuring that the White Balance is consistent among the images and that your Exposure is consistent. Those things should be in place before the stitching. Maybe other things could be done as well, or maybe you could leave more touchups for working on the tiff/psd, that's up to you and the specifics of what you want to do. Whatever you do, though, just have a keen eye to make sure that things are consistent across all your images.