No, you can spend as little or as much as you like at a time. Credits don't expire, ever. They're yours and you decide when and how to use them.
Yes, you can! Just add them one by one on the checkout page.
Yes, you can. For example if you intend to purchase a toy of AED 240, you can top-up your account with AED 220 credits and pay the remainder (AED 20) by card on the checkout page.
Unfortunately, it's not possible to refund credits. But it is possible to transfer credits to friends or family.