I know this works on the FMD and I think it will on the InSight.  If you need a straight section in curved pattern, just hit the pause button.  When you get to the end of where you want "a straight", hit resume.  That will give you a straight line from the pause point to the resume point.  Then you can continue driving until reach to the point where you want to set "B".  I have used this method on the FMD.  I am sitting here looking at the InSight manual and I am not sure if it will work with the InSight of not.  The manual is not very clear, but seems to indicate that the pause button will only stop logging and may not affect line recording.  I may have to play with this some.

I did find in the book where it says you have to drive at least 3 swath widths after setting A before you can set B.  I think it used to say 160'.