I was stupid enough to use an old exported QR code from my Electrum to send bitcoins to, and now I realized the address apparently has changed. The trader advises me the coins have been sent, so they are most likely lost in the ether -- or is there any chance this will get to my wallet eventually? I'm asking this because my Electrum is still syncing and I'm suspecting the coins might be climbing the blockchain-ladder to arrive in my account later?
If the address a wallet shows changes, that's OK. When you look at a wallet's receiving address at different times, unless it's a poorly designed wallet, it'll show different addresses. To increase anonymity, addresses shouldn't be reused, and therefore your wallet has multiple addresses. Don't worry, they'll come sooner or later (but make sure to be using the same Electrum wallet file).