Hiya! If you wanted to upload multiple games onto a single page, then you'd have to just add it on there by yourself manually, so yeah unfortunately the download analytics and views will be lost in doing so.
About the theme, there will be an added theme/limitation announced which you can try and incorporate into your game idea already, on top of making it Halloween themed. It's there just to help people if they are stuck with creating an initial game idea, and we tend to make the theme fit very easily with the Halloween style, for example last year we had the theme "Mask the Truth", which created lots of really awesome games about deception and things like that, which works quite well with a spooky setting.
However if you don't want to use it then that's completely fine too! Jams themes are just there to try and provoke new or unique mechanics but as always you don't have to use it if you don't want to.
Hope this helps! - Mochii