Just to try and cover all the bases, I created a new testing server on a different PC yesterday and spent a chunk of the afternoon testing it. Started over putting fresh exports of RCS output there.
I understand this is getting tedious to follow but here's what I have so far: The issue with the blank slide erasing the pre-existing slide occurs in a slide made using the default template called Blank DDP. If I start from the template called Present Animated I can add and remove slides at will, no problem.
If I use the Present Animated template as is and export it to be embedded, then copy the code that is presented to the head and body sections and move the directory of files to the root of the site, I get a functioning page with the nice slides that are in the template. Yay! So I'm putting the directory of files created by RCS in the correct place on the server.
Great so far, but if I remove all the slides but the first one from the Present Animated template and go through all the steps of exporting to embed again I get no slide and no animation.
Since this all started with me trying to add a single slide to a page on a functioning website, it seems perhaps I've found a bug: RCS can't create a single slide presentation, at least with the templates I've tried so far. Is anyone else seeing this?