I've found that the simplest way to share secrets is through https://sharelock.io/new

Do not expect complete security-- the apparent ease of use comes with a lot of caveats. Ref: https://news.ycombinator.com/item?id=9110146

Then there's the excellent magic-wormhole for the more enterprising amongst us: https://news.ycombinator.com/item?id=14649727

Signal's disappearing messages is probably a better but different alternative to sharelock: https://signal.org/blog/disappearing-messages/

I've found https://github.com/warner/magic-wormhole really useful, although it only meets 1/3 of your requirements. I'm hoping someone will add a nice GUI and re-transmission.

Previously on HN: https://news.ycombinator.com/item?id=14649727