according to http://therion.speleo.sk/wiki/doku.php/exportselectionformat this happens as centreline (polygon draws) and map drawings are fundamentally different, if I understand this correctly; is it possible there is no way to export them 'layered', I mean the map-scraps translucent over the centerline? I would like in some circumstances to have it printed within the resulting mine plan.
This is easily done. You need to define a map with just scraps and define a separate map with just centreline(s). Then select both of those maps for export (or combine those two maps, and select the combination map for export). I see the table on that wiki page is a bit confusing and contains an error on the map - map - centrelines line. I will try to fix.
Another question: I am putting line points over the background image centerline 'wings' (l+r). Is there, perhaps, a function I do not know of which sets the line-points automatically over the centerline left- and right-wings?
No, you have to draw the walls manually for pdf output (although Loch will display the LRUD as smooth walls). Bruce