I believe there should be a limit of one prize per person per month - simply to encourage many people to post on the forums. If one person (no names mentioned) wins all three prizes every month, then others might stop posting quality posts.
To get any discount from Amazon, I believe you have to buy £5000 of vouchers in one go. Otherwise they are sold at the full retail price (in which case, members might as well just take the cash and spend it at Amazon or wherever). I don't think imutual currently has the need to purchase £5000 of Ama...
I don't know of any other cashback site that will resolve claims in less than 8 weeks. It is certainly claimed by some sites, but in reality I've found the "quick resolution" is simply just to reject your claim.