Please do not trust Google Maps - they are notoriously wrong.
In the West it has routed people on nonexistent roads, private ranch roads, you name it.
It may take more work, but there are resources out there.
Years back I did the C&O in the summer and it was great - quiet, cool, adventurous.
But my roommate did it in the fall - when it was rainy - and was miserable.
As early as you are planning, all the water will be turned off, and the towpath will be a sea of mud.
I know you have talked about short daily mileage - and a six month window.
But March is just too early for this part of the country.
Not to mention what a pain it is to get across Chesapeake Bay.
Have you considered starting some place like Sapelo Island, Georgia? (Or Jekyll Island)
Then head across southern Georgia and southern Alabama - many civil rights sites.
Then grab the Underground RR ACA route -
Then working your way northwest up to St. Louie?
At 25 miles per day with every 5th day off - it would take about 9 weeks.
Not to mention lovely azaleas and dogwoods - and very few climbs.
PS - According to Rand McNally it's 827 miles from DC to St. Louis, but only 802 miles from Savannah to St. Louis.
And that doesn't include the extra 80 miles in the Delmarva peninsula plus crossing the bay.
Take away - it is no further leaving from the Georgia coast and with far better weather conditions.
Sapelo Island, Live Oaks and Palmettos