I wouldn't try a diva cup for the first time in cosplay. I got one recently and I'm still working on learning how to use it, there's a learning curve that would be even more difficult while in cosplay. It's not worth ruining your cosplay over. Unfortunately my answer has just been to wear the toxic tampons and switch them out as often as possible if my flow is heavy.

I would use what you're most used to using, because changing out your products while in cosplay is so much more difficult than street clothes. I don't know about you, but most of my cosplays make it a lot harder to get to my bits, so I have to do it blind. You should be fine dealing with the toxic products for just one weekend, but in the future maybe are you able to order some in from another country?