Hey! I can help with the second and third items, investigating the first one.
For purchase flow testing, we have a doc we are working on getting uploaded on this topic but you are correct, if you are making purchases in the dev and staging sandboxes they should not be charged real world money.
What we do not have today is the ability to reset entitlements after purchase. For consumable goods, this isn’t too much of an issue as part of the intended flow is to have the game redeem those entitlements. Durable goods however, would be one and done once claimed for the time being. It is a feature we are looking to add in the future in some form.
For your third question, you could use EGS checkout to sell offers defined in the EOS backend on other storefronts. It basically just loads and routes that purchase through the epic storefront in game and could be used for that purpose. Just make sure you are still logging in with an Epic account and loading the overlay as those things are required to have the checkout flow work in game.