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Piperazine (Wazine is a brand) is effective against certain types of roundworms; it has no effect on other types of worms. Ivermectin is a very broad spectrum wormer; about the only ones it is not effective on are tapeworms. In general, piperazine is considerably milder (less effective) than ivermectin. If the bird has a heavy worm load, you should first worm with piperazine, then a week or two later worm with a more effective wormer such as ivermectin or fenbendazole.
Ivermectin is effective against mites, lice and most biting bugs as well. It will not kill the eggs, however. Depending on the number of birds you have, I usually recommend baths in a dog or cat flea/tick shampoo that contains an insect growth regulator. Pay particular attention to removing the eggs and/or plucking the feathers near the vent. Thoroughly spray the coop with an insecticide designed for animal premises. Take particular care to get it in all cracks and crevasses. Replace bedding and mix in sevin or permethrin dust. Make sure nest boxes have sevin or permethrin dust mixed in as well. DE is an absorbant, and will help keep the bedding dry. It may help with insects by dessicating them. It has absolutely no effect as a wormer. It is safe to feed, and wil result in dryer poop.
The reason for the typically given withdrawal time of two weeks (for eggs) is that that is about how long it takes for a hen to develop an egg. If a medicine is effective in the birds body for longer than two weeks, then you need to increase the withdrawal time to correspond. Assuming that it is something that should not be ingested by humans. Personally, I would rather eat an egg that might contain ivermectin or another wormer than one that might be infected with worms.
If you want to avoid withdrawal times, consider using eprinex, a closely related product that has no withdrawal time.
