Mist downloads new blocks upon a restart, thus is able to receive Ether, but I'm currently unable to send any Ether from any of the wallets in Mist.
I believe the issue is the lack of synchronization while Mist is open, and that my inability to send Ether is a symptom. For example, I've currently had Mist open for 15 minutes and it says "15 minutes since last block"
EDIT: I seem to have bypassed the problem, thus have solved my immediate need (sending Ether), though others may encounter this as well.
What I did was to change the network setting (top menu > Develop > Network) from "Main Network" to "Testnet" and then back to "Main Network". Restarting Mist hasn't fixed anything, but this 'restart of the network connection' seems to have made the difference. Mist is now receiving new blocks in real time.