To answer a few questions:
From what it sounds for you the walk is best done from Brindle Creek picnic area to Selva falls and return.
There is 250m of elevation gain from Selva falls to Antarctic beech.
Swanson falls is now rarely visited as the track is all but lost to the rainforest. I visited it late last year, and while worth it for myself, my hiking partner hated me for dragging him through the thick untracked rainforest!