Hello
@IronicBeauty! You've gotten some good answers from oshizu, Tynynounours, and HelenP, but I'll chime in anyway since I finished the AC recently.
1. if you can only use the fire pit to grill fish to begin with does it have to be surrounded by four walls and a roof?I believe everything except a mailbox, trashcan, and rocket ship needs to be surrounded by four walls and a roof until Space Ranger is cleared. And until Secret Agent- Villian is cleared, all those things have to fit in your 8x8 square (multiple levels are allowed).
2. If you can only use the fire pit outdoors then is using a grill indoors the only alternative with a sim that has at least lvl 3 cooking?I placed a grill indoors, but many others have successfully used grills in common spaces around the neighborhood. I just didn't want to walk so far to grill. The grill is a 1x2 object, though, so you're stuck with it until bodybuilder is cleared.
3. Having the Granite Falls vacation pack means you must only grill fish till the culinary restriction is lifted? Correct. And since grilling fish requires level 3 cooking skill, you have to decide how strict you want to be. Either you can allow your sims to grill fruit (and place a fish in family inventory each time) or not. I chose not to allow it. My work-around was to have my founder join the writer career after finishing his first career. You can only get credit for completing one career per sim, but Pinstar said there is nothing to prevent your sim from quitting the first career (or aging out of it for a teen career) and taking another job. So I had my founder join the author career later in life so that his work task would be "write books." Then, he could write skill books for future generations. You could also live off produce for a while (can't grow it, but can collect it in the wild) but that would be really inefficient and not great for toddlers.
This particular restriction is the reason a lot of people have their founder max the culinary career first. Also, maxing the culinary career requires level 8 gourmet cooking skill (I think) which is difficult without skill books and without being able to use a stove.
4. Is there any restriction on who can unlock a career in the household? Nope, just one per sim as oshizu said. And there are restrictions on moving sims in and out of the household which are tied to certain careers.
5. Having as many children as possible is ideal in order to unlock as many careers as possible till you max the household limit? This is true to a certain degree. My first couple had four children, but the next generation only had two kids. Two is all I had room for at that point because I had too many older sims still alive. If you unlock Diamond Secret Agent early, it would help.
I think your list of new restrictions for the new careers looks good. I only used the original 25 careers because that was enough for me. The only suggestion I have would be to try to tie new careers to some new feature that also came with an expansion pack. So maybe you could say "no clubs" until "X" career is unlocked. Or "no lots traits" until "Y" career is completed. I think I read another AC story where they didn't allow aliens to move in and alien babies from abduction had to be given up until the Scientist career was unlocked.
Good luck with your challenge. I'd love to hear how it goes!