The problem with generating random numbers is that if there is not perfect communication between nodes, there will be two winners. Using hashes always has only one winner, even if the network produces two blocks, the smaller hash will be considered the correct block.
For the ticket to be considered valid, it is necessary to hold 50 coins in the address that will be used to make the ticket.
I think it is also possible to use multisignatures to do a kind of delegation. Using the bitcoin example, it is only possible to have 420000 tickets.
The "ticket" is actually a message that contains the previous ticket hash, public key and signature. So it is not possible to fabricate a block in a way that manipulates the result. As it is not possible to predict the signatures so it is not possible to predict the winner in advance.
Ethereum's implementation is very complex.