need some clarification for some scenarios, I'm not sure if they actually exist in the game but I made them up as examples.
R = river
M = mountain
Ability 1: RRM
Ability 2: MM
can I gain a card that is [RMMM] by applying Ability 1 then Ability 2?
Ability 3: RRRM
Ability 4: MRRM
could I choose not to apply the mountains in Abilities 3 and 4 to gain one or more cards with a path of [RRRRR]?
You are correct in both cases. You can decline to spend symbols, but if you use them, you must use them in the order specified by the ability.