@oshizu I've now completed the challenge and yes it will drive you nuts but it's fun at times.
1. The offerings are like a goodwill gesture, that will bring a celebrant to your area if accepted, if they aren't accepted nothing happens.
2. nope, more celebrants don't show up but use those 6 members to ask the same celebrant one after the other for a sugar skull, it speeds up the collecting a little bit as sometimes 3 out of 6 get a new one.. it's a random skull each time.
3. In my 8 sim household I've found that it doesn't really boost the chance, but it helps with Grim later on.
3a. they are all over the place, I was able to pick up a couple of sugar skulls while two of my sims were on a date in Willow Creek neighborhood.
4. it's random where they show up but after dark in popular places is where I had the most success finding them.
Hope that helps a little.