Yes, 100 is the minimum for seed saving but if you are going to do it seriously long-term, at least 200 is what you'd want. The book Beautiful Corn is a really engaging look at corn's unusual pollination and just everything about this wonderful plant, including an alternate interplanting scheme for cooler areas, using greens instead of beans and squash.
I never thought of it before, but your mentiion of alternative crops for success prompted me to notice that the three sisters each have a different pollination mechanism, which does help to make sure that they don't all fail at once... Corn is wind-pollinated, beans self-pollinating, and squash or melons are insect-pollinated. So it really helps with melon production to have some flowers blooming before the melons do, so your pollinators are already there. Flowering shrubs are good, and kitchen herbs are very good.
Bees love poppies, which are quick. Cilantro is one of the best attractors, like most of the umbelifers. Alyssum is quick and easy and attracts tons of beneficial insects besides
bees. (Is there a beneficial insect
thread anywhere here? That's a pretty interesting and cutting-edge topic. Bountiful Gardens has a collection of different seeds for attracting them. Bees too.)
I found out recently that squash have a
native bee that evolved with squash in Mexico, and migrated to North America with the squash as different Native groups planted it. They live in little solitary burrows in the ground, and are killed by tillage. They like a bit of bare ground near the squash vines. Once I learned about them, I realized that was what made the mystery holes in my garden on bare soil. The holes look like they were made by a 1/4 inch spike, just a round clean hole with no pile of dirt around it. Your niece might want to look for them and know that they are polllination friends.