Yes, the second bucket with the crumbs in it is exactly what you want. Water your plants with that liquid until it's all used up (not all at once, but each time you do a normal watering). If you can get some of those pieces of the dunk on top of the soil, that's good as it will keep working with each watering. Just hitting the plants once with the solution usually isn't enough, especially if you have a bad infestation. It will take some time for it to work, maybe 1-2 weeks, but I guarantee you that it does work. It's not instant death, but rather killing off future generations. Tons of people use this stuff. In the meantime, get some sticky paper or fly traps to capture the adult flies that are in the tent. They're pretty much harmless but it's still annoying to see them there.
And I can tell you from personal experience that fungus gnats won't destroy your whole grow. Yes, it's not ideal to have them, but it's a very common issue that lots of us have. Before I figured out the dunks method, I fought them from beginning to end on my first grow. I still got some great bud from those plants. I know it's hard, but try to relax and remember that these plants are pretty tough and they will fight to survive. It's normal to have some speedbumps in your first grows, but it's all a learning experience.