How does one add a new html-script to ones wiki site?

? Drag .html page file to what part of wiki page? e.g. roll.html or speed-bot.html

Drag and drop a file from the desktop to upload it to an asset folder on the logged-in origin. This means you can only upload to your own sites. They go to a folder on the server that is accessible from the web. Assets plugin markup determine what path will be used.

? scripts.fed.wiki with "Upload" button. How does one add that Upload option to their wiki site? I found it on Ward's and cloned it. The uploads appear to work. But how can be add the Upload support directly?

The Upload is a feature of the Assets plugin. Upload button and drag-and-drop are two ways to access the same functionality.

? Could it be added via the Plugin "+" box's HTML? If not, should that be added?

The Assets plugin is one of the options offered by the new item Factory menu.

? How can related assets be added? images, .js files, etc? e.g. for e.g. roll.html or speed-bot.html

In general, the author of a page sets up the Assets folders for all related assets. There is a hard limit on how big an asset can be uploaded. A softer limit is the backup burden that each asset adds to the server admin's backup responsibility.