Can I mount external SD card read-only?
I disagree with Liam's answer.

Its possible with root, you can just use the "root explorer" app on play store and navigate to /storage/extSdCard/ and on the top, click the "Mount R/O" button to mount the drive as read only.

Surely if you know the linux codes, you should be able to make it into a widget for it :p (one line or so)

(I think there's an app that makes widgets from busybox codes) - that could be your answer