Barry isn't the only lover you can get, but you can only have one at a time. There is another one you can get by choosing the right talking options at Luuk's party.
There are 18 playable characters available at the moment besides Casia. Some are easier to find than others.
The evil tree is indeed in the forest with the snakes. It looks a lot different from the other trees. It's slightly smaller and has two arm-like branches. It's approximately somewhere at the upper left area of the map.
The quest journal is incomplete in the version you're playing, but I have already finished it so it'll be complete in a new upcoming version at some point.
