When publishing from Lightroom to Facebook, you are always sending them to an album. Lightroom doesn't generate you posts though. Generally Facebook is smart enough to create just one post for all of your images published to a single album. If you are getting individual posts per photo there isn't anything you can do from Lightroom's side to change that. Try refreshing you page your newsfeed or timeline to verify whether individual posts are being created or if it's just a timing issue.

Also, uploading images to Facebook will always result in your file having a different file name on the server and in the page code. No avoiding this, no matter your method of upload.

I wouldn't think that adding a watermark would add any significant time to creating the new file that gets uploaded. You could try uploading a couple files without any changes (no watermark or develop settings, etc) and see if that makes things work differently, but I wouldn't expect it.

Try closing all other applications when you start the upload, especially anything that either generates or monitors network traffic. I would expect that to be the choke point that might cause an issue.

I messed around with this a bit. I found that if you are watching the page during upload and continually refresh the page, the first couple images will post individually, but eventually they will be grouped in the Album as a single post. I did notice that if you modify the the information on the image, such as the date the image was taken, Facebook will retroactively pull the images into individual posts based on date. This probably has something to do more with Facebook's new Timeline layout than anything else. There isn't anything in Lightroom that actually causes this to happen. It's just the way Facebook is handling the images.