Would it make sense to grey out the “delete polyline” option unless the “from polyline” option is selected? I know this led to some confusion for me.
Hi,
I agree with that an unavailable option should be grayed out e_geek
Other issues:
A: I understand that the visible frame is governed by the attributes.
Still I have a question with the used color and Linetype when not visible.
Similar as auxiliary shapes would be appropriate, using a continuously polyline pattern too. 
B: This placeholder is also printed in some cases … e_confused
It depends on the order of things.
Setting the Wipeout on a non-printable layer is not really an option
Regards,
CVH