Can I use dumbbells instead of kettlebells?
Well, yes, but no.
While it's mostly true that anything you can do with a kettlebell can be done with a dumbbell, there are some subtle differences between movements. For example the way the kettlebell handle is designed makes it rotate great in your hand, and also sit differently in the rack position. Think of doing a clean with a kettlebell and doing a clean with a dumbbell, there's a difference in the bell path and finish position.
The snatch is another example of subtle differences. Kettlebell snatches are done from the hang essentially, as kettlebell movements are usually initiated with the bell passing between your legs, and being driven out by the hips. A dumbbell snatch however, is done from the ground.
Personally I find kettlebells to be more satisfying to use and dumbbells to feel more clunky. Each has their own strengths and benefits.
When you say swing, do you mean overhead?
Mentioned by Pavel in his original book Russian Kettlebell Challenge and often referred to as the American swing. When I say swing, it's what I mean, sometimes I'll specify Russian swing.
Personally, I find them extremely beneficial. Heavy overhead swings cause so much hip drive, my feet come off the ground.
Somehow it's very dogmatic, it seems like people are either against them or for them. I'm not going to try and convince anyone otherwise. Do whichever you're comfortable with.
When you say for time, how fast is that?
Go bells out, as fast as you can, think of running a race.