Hi!
Please put your MTB routes into relations ( see https://wiki.openstreetmap.org/wiki/Relation:route#Cycle_routes_(also_mountain_bike) ) and don't put the oneway=yes on the individual segments. The advantage of using relations is that you can reference existing way segments without the need to create overlapping/parallel ways. And I'm sure your way segments are no oneways. When using a route relation the correct way to map a preferred direction is using the forwards/backward tag on the route - see https://wiki.openstreetmap.org/wiki/Tag:route%3Dbicycle