They are actually really easy to sex if you flip them over. The male abdomen flap is really narrow and pointy compare to the females. I think most pet stores will usually carry geosesarma hagen or geosesarma dennerle but I also seen multiple species mixed in one single tank before.
Yes stress is always a big possibility, imports might have gotten better over the year but I remember when they were first on the market the survival rate was incredibly low. Also their lifespan is about 2-4 years and since they are wild caught there's no way to know how old they are.
I do hope you try again when you are ready, they are honestly really fun to keep, one of the few crabs that give birth to fully develop baby crabs that do not require a saltwater stage. They are also really cool hunters that will pounce on fruit flies or baby crickets given the chance.
Feel free to PM me, there's a few groups that are dedicated to keeping these gems that can probably provide more information if you decide to keep them again.
I believe there were three males and three females. As far as I know, they were the same species. They were just labeled as vampire crabs at the store, so there were no more details than that. If I do decide to try again (still undecided - I don't want to risk hurting more) I will get a bigger tank and provide more land and hiding places.
So it's possible that they died from the stress of travel and re-adjusting?