1) Add new hero please!
The heroes that are currently not available in epic boss fight are entirely reliant on hero interactions and would not fit a PvE gamemode or they have game breaking bugs in their abilities. Either way they will require complete reworks and WILL NOT be added to EBF in their current state. They will be added as a new hero like Elementalist, Skinwalker and Mana Fiend. This means they are not really a priority to be added and will be added whenever it strikes my fancy.

2) Game won't update/custom archive doesn't match/etc
Try uninstalling and reinstalling, otherwise just wait for valve's amazing custom gamemode system to fix itself.
Go to your steam directory; SteamApps\workshop\content\570 and delete this file: 305278898; this will properly remove epic boss fight as the ingame uninstall can be buggy.

3) Give leaver gold!!!
There is a system in place; you receive bonus gold per hero abandoned/disconnected for longer than 5 minutes. I am merely not removing gold from the dc/ed heroes in case they come back so they aren't hopelessly behind.