I think it is only difficult to see where you are and what happens. If an apple spits a seed standing next to you, you die without knowing what happened, for example
And - yes, Metallic Water is one of the hardest quests.
And of course you can complete all quests (Imagine JoeKickass would complete Best Player - it is the last quest missing for him (plus Metallic Water and Drunk Man... But to end Best Player, he must end these quests first). There are some players on the french server who have done this, but... this is not the end

You can continue playing as long as you want. Trying to get more points or to unblock all coeff 5s, or... there are enough unreachable goals^^