3. If you roll Force Dice as part of a skill check all force pips can be used as successes or advantages at the discretion of the player. So, the check only fails if all successes + force points don't exceed the failures. If you roll a skill check as part of activating a force power then the force dice just determine if you can must the strength to activate the power while the other dice determine if you use the power successfully. Force attacks in particular tend to have two points of failure, and are always opposed checks.
4. You can flip a destiny point, take conflict and strain to use a dark side point. Technically you can never fail a force check as long as you have destiny points and strain to spend. Refusing to take conflict is a common issue in the game though, because people think conflict is evil points. It isn't. Doing evil things does award conflict, but sometimes doing the right thing also awards conflict. Let's say your friend is falling off a cliff, you could use move to catch him, but you only rolled dark side points. You might say you're not going let the fear of loss control you, and accept death as part of the balance of the universe, and simply let your friend fall. Or you can draw upon your fear of losing your friend and save him. Taking that conflict doesn't make you evil, it just means that you value the life of your friends more than living as a perfect force ascetic with no attachments to anything or anyone.