At the moment we aren't planning to do any more development for our site until we can migrate to XF2 since we'll have to rewrite everything then. Perhaps once we move to XF2 and have fixed all the bugs that come with that and have more time for some new development we can work on this, but for the moment this isn't something that we can justify the development for.The reason you can purchase Supreme even though you already have it is that you've never bought Supreme before, you purchased the Premium to Supreme upgrade and it is a limitation of our system.
The second issue is the same reason that the Premium to Supreme upgrade and Ultimate are not giftable. Due to limitations with the gifting system, it is not possible for us to check if the other person is eligible for an upgrade. I hope to get this fixed soon with a bit of an overhaul to our account upgrades system.
