Hey, welcome to DF! Don't worry about having lots of questions, everyone has them to start, and even the long time players are just getting to grips with the latest version. There are always people happy to help on these forums. Though you should check the wiki if it's a simple question about gameplay, or some of the less friendly people on here will yell at you.
About your questions, I wouldn't rely on the mouse for playing DF. The interface hasn't really been optimised yet due to the alpha state... The developer has decided that the game is likely to change so much that time spent on it would probably be wasted in the long run. Nevertheless, it is still very functional. A major tip that might not be obvious - If you want to scroll faster, on screens like the mining designation screen you can hold down the shift key to jump ten spaces instead of one.
As for quitting without saving, it isn't really recommended but you can exit with ctrl-alt-del and end process. It might mess up your world, but it probably won't. Something else you should definitely do is go to the .init file hidden in the DF folder and turn on seasonal autosaves. If you don't want to risk messing up your game, you can quit and replace your region save with the autosave (the region is the save for the entire world you have generated). Delete the region saves folders contents and then copy and paste the whole contents of the autosave into it.
But definitely turn it on. When DF crashes/goblins slaughter everyone/you irreversibly flood your fortress/everyone starves/fortress caves in/everyone catches fire/(etc.) at 4 in the morning and you've been playing without saving since 10 at night, you want to kill something. Have fun!