Thank you all for the quick responses. I think I have to clarify myself a bit on Qs 3. and 5. I'm sorry, I'm not an indigenous English speaker, I don't have the skill to express myself shortly.
3. So if an Immortal does requirement X to eat Ambrosia, no later Immortal can do that (even just for its sheer benefit or for fun) nor can the former Immortal change his unique requirements in order to let other Immortal have those as well (again for their sheer benefit). Ok, I got that, but I'm still confused about one thing.
My first Immortal has taken 6 LTRs, so he's well beyond the requirement of three unique LTRs. Do I have to choose now which are the ones to qualify for the requirement? Or can I opt not to choose them and just have at least 3 unique LTRs and later have some other Immortals take some of those LTRs as well, as long as the first Immortal has at least 3 unique LTRs? Of course the mutual LTRs wouldn't be unique to the later Immortals either, but they could want them anyway.
LTRs was just an example, the same principle would work for any other requirements, wouldn't it?
5. The reason I have to travel is, because I have to Super-Max a skill. And because he's scientist, he's closest to Super-Max handiness, fishing and gardening, but I just can't decide which one to choose. Well handiness doesn't require travelling but the other two do, am I right? Good tips from Audren and Seabody. So I have to either buy cheap appliances and use them repeatedly and hope they break often enough, or wait till my heir is YA and travel to get all those needed fish/seeds and just hope my first Immortal doesn't die before it. Any other suggestions?
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Did I make myself clear this time?
3. I agree that it's best to make your decisions now and not wait. Your first immortal will have three of those LTRs as the unique requirements. The others may use the other three, but they cannot be unique for anyone. For example, it's ok for everyone to have the Steel Bladder reward, but it can never be counted as a unique reward for Ambrosia requirements.
4. This challenge was designed so that even someone with only the base game could complete it. So, traveling is not necessary for completing Fishing and Gardening SuperMax skills. For Gardening, 19 plants are required to complete the Master Planter Challenge. Regarding Fishing, new fish have come with almost every expansion, so you should be able to catch the 20 required types of fish easily in your town.
One more thing... you've got three possible SuperMax skills: Gardening, Fishing, and Handiness. Right? Keep in mind that if any of them reach level 10, they are disqualified from being a SuperMax skill for future generations. Even without doing all of the journal challenges for them, they are still disqualified just because they are level 10. It becomes like the non-unique Lifetime Happiness Rewards. Other Immortals can max those skills by going to level 10, but they cannot be a SuperMax requirement for eating Ambrosia.
I hope this helps clarify things for you.