1) z-index value of slideshows generated with WOWSlider is 100. I don't understand where you found values "1, 5, 6, 7, 10". Actually, plugin itself has not any z-index value. ZIP file contains WOWSlider files (images, .css, .js) and couldn't have z-index parameter.
Also, maximal value of z-index (for modern browsers) is 2147483647:
So, "100" isn't so "large" value.
Also, there is no standard/universal/default value of z-index parameter for different elements. Each developer can set any value that he wants. Of course, we couldn't guess what "z-index" value has each element on the page of any user and which element should be overlapped and which shouldn't. So, it's impossible to set "universal" value of "z-index" parameter that will be suitable for any user in any case.
If you use elements that was created by different developers, of course these elements can have a conflict.
So, it's not a big problem to change the value of one parameter.