Is throttle-only possible for mid-drive?
While I'm not aware of any examples of a throttle operated mid-drive, it is certainly possible. With a regular bicycle crankset, a mid-drive motor will push the crankset around, transferring the power into the chain through the chainrings to the chain. This involves causing the crankarms to spin while in use, making it Pedal-Assist only for any practical use. 

What you would need is a crankset like the one in several Specialized ebike models. These have a freewheel-like release in them. This means that when you pedal backwards, the chainrings do not spin backwards, but when pedalled forwards, the crankset mechanism engages and turn the chainrings forward. With a crankset like this, you could have a mid-drive ebike which could be operated by Pedal-Assist or by throttle.