To help others who might have been in doubt just like me ... Due to my 'logistics' I tackled the path in the opposite direction from what I asked in the post.
The section D.Trebuša - Stopnik is obviously less used than Stopnik - Reka. The trail here is a bit more 'dirty and overgrown' ...
In short ... the path is ... and it is perfectly walkable. No orientation issues. Only right before reaching the first houses in Stopnik the path briefly completely deviates from the one marked on the maps. The path here steeply climbs about 40m higher up the slope. Obviously the 'original path' was too exposed at higher Idrijca water levels (guessing).
The section Stopnik - Reka is freshly tidied up and cleaned tip-top. Obviously something is happening here (maybe some trail run). Only two fallen trees remain right before the picnic spot, but work is ongoing as the workers left some equipment there.
The only downside, in my personal opinion, is unfortunately that (too) long gravel-asphalt section through Stopnik (cca 2km), which dragged on for me. I'm just more of the rugged type :-)
Otherwise .. from the bridge in D.Trebušah to the 1st bridge in Reka it's cca 11km/400m.
A little more in the photos ...
@Tulipan, Miri: Thanks!