<< Can I take a curved solid and make it flat?

Of course yes!
Use Flow again ;)

Here i use the Flat Line so has easily the middle start curve!

Make the curvated volume by extrude

Draw any staight Line as target withany measure...

(if you want to know for any reason the result of the exact lenght before use the Flow function use the UnWrap curve
https://moi3d.com/forum/messages.php?webtag=MOI&msg=5136.1
Or use the Button Detail of the selected curve! :)

Make the Flow

Et voilà you obtain your Unflow volume Curvated to "Flat" !