the referenced topic is refering to library playlists, not exported playlists.
Trying again now for exported playlists i am unable to reproduce any of the issues you list ie. relative paths works fine, unix convention works fine, export tickbox is persisting.
As to the export setting itself, i cant see any code that disables that setting nor did i have any problems with it.
This functionality has been around for a very long time, so i'm inclined to not spend time investigating it further.
For the last point, when you say "selected from Explorer", do you mean you open the playlist by double clicking in windows explorer?
edit:
maybe if you post a screenshot showing the playlist settings screen where the setting disabled might throw some light on things