Looked at your photos the other day but haven't had time to compare them with your text until now. Looks like you don't have a terribly steep slope there to deal with or particularly long. Not like 100 yds! I can see where you got the idea to just divert the excess water away and into the county ditch. On one hand, I kind of agree with you.
I see your thinking in slowing down the water and in most cases I would agree if the runoff was continuing into other landscaping after being diverted, somewhat like into a rain garden, although that analogy might not be the most appropriate. Not sure I'm seeing where it would hurt though in this case to let it run as fast as it wanted to- as long as the erosion issue would be addressed by a concrete/pipe/whatever lined ditch that definitely emptied into the county ditch and not into any other area.
Each of these ideas has its merits. I suppose it comes down to what MM is able to do and what blends with the landscaping theme over all. If you think about it, RT's creek bed is a ditch, just with slow water. Actually, from the aesthetics perspective I lean that way. So- fast ditch or slow ditch? I don't know what the cost difference would be there between the material choices. To me, landscaping is a matter of taste similar to one's wardrobe.
I'm really impressed with those graphics as well as the thought given in this thread. Lots of other forums have self-serving idiots who, like my daddy used to say- "Better to be thought a fool than to open one's mouth and remove all doubt!"
Maybe a follow-up on this after you decide? Couldn't help myself on the fast ditch/slow ditch thing!