I'm FOR Yen Sid being a Keyblade master. As in, one of the 3 masters from Land of Departure.
I can't see him actually using one to fight though.
My points, as to why I CAN agree with this:
Yen Sid = Disney
Eraqus = Square (I know this seems basic but Nomura did say nothing happens by accident or something, so it could be the reason he chose this name.)
The Star Seeker scene and handing it back is somewhat similar to Traverse Town visit 2. Riku takes Sora's Keyblade but GIVES it back, it doesn't go back to Sora of it's own accord. The Star Seeker did the same thing. In every other scene, when wielded by people who aren't users, it returns in a flash of light.
Sora obtains Star Seeker in Yen Sid's tower. This only goes to support Yen Sid having a deeper connection to the keyblade and Star Seeker.
Mickey needed the Kingdom Key D to be able to lock each keyhole from the Dark Realm. In other words, he would need a keyblade to SURVIVE in the Dark Realm, since PUREBLOODS do indeed originate from the Dark Realm. How would Mickey fight the Heartless without a keyblade to begin with? He went there to find a Keyblade. Yen Sid was Mickey's master. Mickey told AtW of the legend of the Keyblade. How did Mickey first discover this?