I can only comment from my limited experience so far with the Tubus Swing and it is stayer for me at this stage given my bikepacking use. Even on the single-track and on the rutted out climbs (8 to 10% grades) it made a difference to the handling of the bike, by having some weight up front. That said I did find it preferable having the front suspension locked as this helped reduce further the front bounce, particularly on the steep climbs in the pea gravel when the back wheel spins.
I really have no idea as to why it is a "little bizarre" but I am more than happy with the trailer. It works. I really don't seen the point in trying to get a rear rack fitted to the bike when it has no provision for one; more so when I already have the trailer for road touring as well as bikepacking. I could go with a Freeloader I guess but they don't seem to be available anywhere now that Thule has brought them out. I may go down that path in the future but since I have the trailer and it works it is a low priority idea.
BTW prior to the Extrawheel Voyager I used a BOB Ibex behind this bike. That worked well too.
Cannot comment on mountain passes; we don't have them here but so far I have found it handles very well both
on the road and off the road. I generally forget about it until I have to man-handle the bike over a log or ten but even then it is not a significant hassle.
Carrying less stuff is not always so possible here as often we don't have shops readily available nor water. I have carried 35 + litres at times for example. On this particular ride it was not really loaded up by any means. The panniers where not full with the exception of one stuffed with my bulky quilt.
So far and I have done over 3,000 km with the Extrawheel Voyager on bitumen, dirt roads, single-track, rutted out forresty roads, over logs and rocks and it was performed well so happy to stick with it. Your experience of course may be different.
Andrew