I can't answer all of your questions, but I can answer some.
1. Yes, you can buy ones from Walmart as long as there are no additives like scents.
2. I rinse them, but I don't think it is absolutely necessary. I just like to minimize the amount of dirt my frog consumes.
3. Yes, they should still be dusted with calcium.
4. Not sure about how many or how often, it depends on how big they are. You can cut them in bite sized pieces as well.
5. They can last for weeks in the fridge, I've had them last well over a month.

The pooping question depends on a few things. How often he is eating, how much, and your frog in general. Do you see a lump on his side? That is usually an indicator they have to poop. I'll let the experts chime in here.