The Umpani / Trang missions are totally optional. The only thing that opens the way you are referring to is placing the three artefacts.
When you place the third artefact 2 Yellow Dots appear on the map and the way is opened. (Can you see the 2 dots with X-ray ?)
(I assume you are referring to this map
http://www.volberding.com/Wizardry8/...k/Peak-map.jpg )