1: What exactly does it mean to "Secure Your Spirit's Future?"
For whatever reasons you chose, your Spirit was either bannished, lost, trapped or something like that, it now wants to get back to where it used to be or how it used to be. So who or whatever was keeping it away is now gone and it can come back, it still needs to pass some goals to do that, thus securing it's future hold on the real world.
2: To get the ingredients for Ambrosia, is this what you mean? Say, 1 gets money, 2 gets fruit, 3 gets fish, and 4 makes it. And 1 could not grow fruit, catch fish, or cook it?
Correct.
3: What exactly is the end of the challenge?
Sorry, I realised I hadn't actually written how it ends very well, the Fifth Generation is when the Spirit is reincarnated into it's own body, this body then only has to age up to a Young Adult and eat the Ambrosia. You can either have the Fourth Vessel earn the Fridge from the Culinary Career that never lets Food go off, or you can put it in a Food Replicator. If the Fourth Vessel is still alive and you were unable to get any of these items then I would permit you to use the Fourth Vessel to prepare some Ambrosia. Although I wouldn't plan on doing that, you have a few goals to complete and that might take up a lot of the Fourth Vessels Adult Lifespans and may cause them to be on deaths door at that time.
The way I perceive it is that the end of the challenge is when the 5th generation is born.
To "secure your spirit's future" is more of a term. I think it mostly refers to reaching the tops of a career. For story reasons, it could mean that by doing those tasks your spirit becomes strong enough to enter a new vessel.
Ahk, sorry TallStar! I should have realised my own wordings crossed over, I had written 'the Spirit must eat Ambrosia' but then I put 'the challenge ends when the fifth Generation is born' which doesn't give a clear meaning... However you can age up the Spirit at anytime you wish because their only goal is to survive until they can eat the Ambrosia as a Young Adult. Your idea of 'secure your spirit's future' is good as well, it is securing it's future in all of the four main categories of the social rankings, through Politics/Business, Arts, Military and Knowledge.