1)Do you think it is dangerous to lunge without a whip?
Nope. As long as your horse knows how to lunge and behaves while lunging.
2)Do you think you should lunge without slack in the lunge line?
Kind of a hard question to answer. If your horse works good on a loose line, good. If he works better on "tighter" line than that is fine. I would prefer the horse to be on a looser line because that indicates more that he is listening and knows how to go in a circle.
3)Do you think using sliding reins is bad?(in the way I told you I used them)
I am not sure what sliding reins are? Could you show a picture? And in your first post you said you didn't use them very often, you just used them for that particular lunge. Using less "mechanical" (as I like to say :)) is always better. BUT even when using them not often you MUST know HOW to use them. Same goes for all equipment.