From whichever path you like.
Entdecker - Realm of the Desert Sons
No. Only oases with symbols can be rated, and water must only be paid for oases that can be rated.
If 2 oases are completed at the same time, may I take water from one of the oases and apply it to use the highest Influence Stone in the other oasis?
No. If 2 oases are discovered simultaneously, you first have to pay 1 water for each of the two highest Influence Stones.