You should use BigchainDB 2.0.0b5 and Tendermint 0.22.8 (not higher). There were breaking changes in Tendermint 0.23 and BigchainDB hasn’t been updated to account for those yet.
The BigchainDB testnet doesn’t currently allow external users to connect directly to the MongoDB database in any of its nodes. External users *can* do any of the queries provided (indirectly) via the BigchainDB HTTP API. See https://docs.bigchaindb.com/projects/server/en/latest/http-client-server-api.html
Trent is our CTO and rarely ever checks the BigchainDB Gitter channels. The rest of the team checks them fairly often though. Did you use an open public channel or did you send a private message? The main bigchaindb/bigchaindb channel is at https://gitter.im/bigchaindb/bigchaindb