Answering your thread title question: no, The Witcher 2 is most definitely not a sandbox RPG. It's not linear, as well, but it's far from being a sandbox game.
What's great about The Witcher games is their high replay value via a huge multitude of story choices and consequences, no two playthroughs of the games are the same. And since the consequences of your choices come later in the game (often during later chapters), it prevents you from the usual save scumming associated with RPGs, when you want to achieve a certain outcome, immediately, and keep reloading saves until you get it right. This is almost impossible to do in both The Witcher games that are out now, at least regarding some of the big, ambiguous moral and plot decisions you have to make (sometimes even choices you've made that you thought were of no huge consequence will influence the development of your personal Geralt's story).
If you're looking for Skyrim, well, stick with Skyrim, The Witcher 2 is nothing like it and, to be honest, I'm glad it's not. Maybe you'll get luckier with The Witcher 3, at least it's open-world, though I'm not so sure about the sandbox part of it, as CD Projekt RED has stated they plan on keeping with their highly acclaimed stapled story and character development of previous titles, and that kind of clashes with the notion of a "full" sandbox game.