Aerospike touches most of your points at much higher speed and scale. It is next gen redis, basically, with disk, with auto-sharding scale, with cross node queries. Cassandra is not difficult once you wrap your mind around column storage, and if you need that, no other storage style will do.

The 15-node thing is also a major achilles heel. Who wants to commit to a stack that incurs massive technical debt in the event of massive success? Imagine reengineering your db and your event pushes, at scale...