a database noob question, when you say distribute the database to 50 servers all these are shards, correct? MySQL (or any standard RDBMS) can't span more than one server, correct ?
Yes, these are separate server instances that are essentially separate from each other. All of the records for a given user live on a single particular server. Although MySQL doesn't do this kind of partitioning inherently, this kind of partitioning can be implemented in the application.
Beyond CRUD, I'm not so sure, and any application that's big enough to use sharding probably has some pretty complicated queries. At that point you really don't want to be depending on a library to handle the low level details. You need to think about the performance implications of the access patterns and the trade-offs inherent in different design choices. If a library makes these choices for you then you are unlikely to get optimal results.
Interested to know how you'd avoid duplicates and lost transactions while maintaining a comfortable level of performance/latency for an online game.
Read-only replication stores a copy of the whole database on other machines, so that any query which doesn't write can be handled by asking the remote machine. Writes are bottlenecked through the primary machine, which then sends changes to the remotes.