How much does it set me back to miss a review once?

WK either promotes or demotes an item based on whether you get it correct or incorrect. So if an item is at Apprentice 3 and you get it incorrect, it gets demoted to Apprentice 2 and now has the SRS interval for Apprentice 2. It does not accelerate the time.

There are some userscripts that some people use that allows them to undo typos on the spot, you can check those out if you want.