Changing speed technically could effect stability if one of the players couldn't keep up. The game may start to lag or mini freeze if speed is too high. But its upto the players as to what they do or don't do.
The community maps folder works though currently people have to manually place owps there, in the future the concept is to use the workshop. There is no plans for OW to download maps from the host.
I have no clue what would currently happen if a host switches to a map which a player doesn't have. If they try to connect to a game which they don't have a map it should stop them from connecting. Its not really been tested fully by the community.
A long change list for one patch.
Its 220.127.116.11 -> 18.104.22.168 changelog as this is the patch that has been sent to Bohemia and will (Unless they find any game breaking bugs) be the next released patch.