Is there any possibility of uploading a demo of only one product page without destroying the rest of the web page and without overwriting all the information that I already have right now?
You can simply import that page, simply export that page from your other website and import in this website using import option from WordPress tools menu.
I have bought the theme, are there different simple product pages to choose from?
Could I look for a simple product page and upload it to my website or are they all like the one I have installed?
There will be same in your demo which you imported. Those will not be different as product pages do follow a unique template for whole website. Let me know if I did not got you properly or you want something else ?
