I'm kind of at a loss here... If you have imported the private key (not seed) and it shows that exact address, then it should show the 0.5 BTC balance!!?! 
Time for another stupid question... You have checked the ENTIRE address for correctness... Every single character? I know I'm bad and usually only check the first 4 or 5 characters and the last 4 or 5.
Can you tell I'm grasping at straws here?
Maybe see if you can create a sweep transaction? In Electrum, goto "wallet -> private keys -> sweep"... Input your private key (and a receive address if you want to send to a different wallet) and see if it says "no inputs found" or it comes up with a "send" dialog...
Other than that, I'm not sure what else to try?
Time for another stupid question... You have checked the ENTIRE address for correctness... Every single character? I know I'm bad and usually only check the first 4 or 5 characters and the last 4 or 5.
Can you tell I'm grasping at straws here?
Maybe see if you can create a sweep transaction? In Electrum, goto "wallet -> private keys -> sweep"... Input your private key (and a receive address if you want to send to a different wallet) and see if it says "no inputs found" or it comes up with a "send" dialog...
Other than that, I'm not sure what else to try?