I've had a pretty good success rate with my mimosa seeds. I use some tiny, sharp scissors to snip away the shell almost all the way around the edge of the seed. This is incredibly tedious and fiddly work, but allows the embryo to expand during the soak. Just nicking the outer shell isn't enough. These are tree seeds with very tough casings so it's important to free the embryo as much as possible to promote maximum hydration.
Next I soak them in a thermos of lukewarm tapwater for a couple of days (most seeds will swell during this time), then wrap them in a doubled-over wet (sterilised water) paper towel, which goes in a clean baggie to be stored in warm darkness for a week or so.
In my experience with this method, you can expect at least 5 healthy germinations out of every 10 seeds. Some I've taken out of the bag were already inch-long seedlings - those were the ones I kept, and I threw away a dozen or so due to space constraints.
Keep trying. I don't think it's beyond anyone's realm of possibilities to eventually cultivate their own private mimosa crop.
Sometimes I believe that this less material life is our truer life, and that our vain presence on the terraqueous globe is itself the secondary or merely virtual phenomenon.