I joined a table Geronimoo was hosting right before he started the game. He didn't realize he wasn't playing against a bot until I took my turn, and I ended up resigning because he wasn't looking to play against a human.
Adding a "ready" button that all players have to click before the game starts would help. (This wasn't an issue in the MF client because you had to add bots manually, but I like the current setting of adding a bot automatically, and would like that to stay.)
Yes, this is already implemented in the dev version.
Yes, replace the label for the table owner to "start game" and every other human player who hasnt clicked ready yet gets replaced by a bot (in case of a last second join).
Also I prefer to choose the number of players right from the start 3 players instead of 2-4. That would make me more comfortable joining a game if I know in advance the number of players it will consist of.