Mystara Without High Technology
by HåvardMany have pointed out the kitchyness of having Star Trek like elements in Mystara's past and present. If you like these elements, fine. If you don't, getting rid of them in fact isn't that much of a problem.
Imagine the Radiance as a Monolith, not unlike the one from that movie whose name escapes me. The Radiance Monolith is discovered by Blackmoorian adventurers exploring the mysterious Valley of the Ancients (ZGG has indicated that they intend to have something other than a spaceship there also). The discovery of the Monolith leads the Blackmoorian wizards to develop their magic and build powerful arcane devises. In this development, something eventually goes wrong resulting in a magical disaster known as the Great Rain of Fire.
Several thousand years later, The Radiance Monolith is rediscovered below Glantri. This eliminates quite a few problems, though we are left with at least one.
What now with Benekander? Well, Benekander doesn't necessarily have to be an alien.. He may well have been one of the wizards of Blackmoor studying the Monolith. Benny was sucked into the Monolith for unknown reasons and was only released as an immortal during the GRoF. That should solve most of the tech problems with Mystara. If you want Benny to be from a different world he may still be. The Monolith most likely came from Space, and may have visited other worlds before it arrived on Mystara. Perhaps Benekander discovered the Monolith and was sucked into it while it was visiting a world populated by Grens, travelling along with it until his release on Mystara, either during WotI, or he may temporarily have been released in the age of Blackmoor, although this isn't really necessary.