Is there a way to rearrange the sites in the "Manage Sites"?

There's no way I know of to change the alpha-numeric order DW uses, not from within the interface anyway. Renaming the sites to take advantage of it will do the trick. Add a number in front of each site's name...

01 MyAwesomeSite
02 TheAwesomeSite
03 ZebrasAreHorses
04 AdobeStuff 

...and they'll be listed in the order you set.

1. Go to Site > Manage Sites
2. Click the site you want to rename to highlight it
3. Click Edit
4. In the Site Name field, change it from "YourSiteName" to "01 YourSiteName"
5. Save
6. Rinse and repeat for every site, in the order you want them