Theres is alot of reaserch on the impact of the Cairngorm reindeer. They are a native species, its just wrong to say otherwise, thus their return should be studied (which it is). Alot of this reaserch uses inaturalist to estimate the range of the reindeer. Whilst the charity does ofcourse manage their population they dont always know how far they travel.
Yes, without human intervention they would have never died out in the first place.