If youth members are added as attending an Adventurous Journey will it come up into their record and can this be recorded as assists, leads and participates?

If the Adventurous Journey is planned as an activity through the Programming function of Scouts | Terrain, then yes absolutely youth members can have a Participate, Assist or Lead credit added to their Achievement Pathways, once the Adventurous Journey activity has been concluded in Scouts | Terrain.

This involvement will credit where applicable to their Milestones.