My understanding is it's very different. Date Taken is automatic and set, by the camera, to the time the photo was created. Release Date is the (usually) commercial release date of a finished, edited-together piece of media, and usually set by hand (if set at all). It'd usually be the cinematic release date, if it's a movie that was in theatres.
Of course, the fields can be set to any value, so some people or workflows may use them differently.
Yes. Here's How to install script add-ins using the ColumnRelaseDate2.js.txt attached in Lxp's post.
After installing it, you'll have a new column in the Scripts category which you can turn on and off like any other column. (Or you can use the Set COLUMNSTOGGLE... commands Lxp provided if you want to create a button or hotkey which toggles the two columns.)