For the first question, just select all the text in the script editor then drag it up to the shelf using the middle mouse button. Presto! - a new button.
Couldn't find any info on a reset option for the panel layouts though. Guess it's a matter of deleting all the layouts and recreating from scratch, unless nuking your prefs directory will reset everything to defaults.
EDIT: Was just looking through the help, "Quick layout buttons" tells you which mouse buttons do what when over the panel icons. There's even a picture of the default setup there so you can put it all back together. But still no joy with the reset option..