To be honest, it's a bit difficult to understand how the planner works from the manual. Not the manual is poor, it's just difficult to understand, until you study it for quite a while.
I just did a quick test. Increasing the corner error does exactly what I'm asking for between two G1 moves. If I input a corner error of 25mm, then it will round off the corner by up to 25mm, if the acceleration is low enough.
Why can't you do the exact same thing between a G1 and a G2/G3 move. The corner error doesn't seem to effect these moves. All we are asking is to basically have the corner error do the same thing on G1 to G2/G3 moves as it does on G1 to G1 moves. Nothing more.