I've been doing some reading recently on George Washington and this question came to mind. Did the Continental Congress ever have a solid plan of what they would do should the Commander in Chief be killed or captured?
I understand that throughout Revolution, there seemed to be a revolving door of "2nd in commands" to Washington, and they all seemed fairly incapable or possibly even corrupt (i.e. Charles Lee and Horatio Gates). Would it have fallen to someone like Nathanael Greene or Benjamin Lincoln?
There wasn't a clear contingency plan, but they wouldn't really need one. As you've mentioned, there was a pool of Major Generals one rank beneath General Washington. Upon Washington's death, command would pass to the most senior Major General until such time as Congress named a replacement.
Between 1777 and 1780, the most obvious choice was Gates, the Victor of Saratoga. In fact, until his disastrous defeat at Camden, Gates had a better service record than Washington did, and there was a considerable movement to fire Washington and replace him with Gates - a move backed, at least at one point, by John Adams.
After Camden, Greene probably would have been the most obvious choice. He did well against Cornwallis, and Washington entrusted him with command of the southern half of the Continental Army after Yorktown.
Source: Robert Middlekauf, The Glorious Cause