A quick glance at the outputs and comparing with the xvi inserted in the th2, for blackarrow at least, suggests your scrap is drawn based on an xvi extending one way, and a centreline extended the other.

If that is all that is going on, then -flip horizontal is what you need in your scrap definition.

It seems likely that there is something else, and I would hazard a guess that the extended centreline generation differs dramatically (the 30 m offset you mention) from the xvi that informs the scrap drawing.

Have a look at https://therion.speleo.sk/wiki/breakingextend  and https://therion.speleo.sk/wiki/extend

My experience with extended elevations is that starting when the survey is small and simple reaps rewards, as does painstaking care that the extended centreline generation is nearly perfect before you start drawing scraps.

Bruce