@ShellShock - you have to make sure you restart calibre after installing a plugin. I suspect there was a combination in there with your multiple installs where either you missed that step - perhaps installing two new plugins for the first time without rebooting between it creates an issue. Kovid has "dabbled" with that code in a recent calibre release to ensure that by default it tries to put the plugin on the toolbar for both when a device is connected and when it is not. You will want them on both toolbars ordinarily, or else you will be joining the queue of people posting why their icons have disappeared...
As to your suggestions. Normalising of dashes from -- to — will happen automatically if you use the "Smarten punctuation" option of the plugin. As to the other dashes that look almost like emdashes etc - I'll be honest and say it isn't a huge priority personally. Perhaps you could try asking Kovid to enhance the Smarten punctuation logic to incorporate some of them if he is interested, then you would get it for free.
Not sure what you mean by fixing cover image page backgrounds? Do you have a book example? PM me a link. Not something I have ever noticed, but then I always run with the defaults of calibre stretching the image so a background isn't visible anyways. I would consider it unlikely that I would do stylesheet dabbling in this plugin but if you can send me an example I will take a look.