Strange that the form didn't work... I wonder what happened. Thanks for reaching out on the forums though.
Yes, you can register .de domains with DreamHost.
I'm not sure what you mean transfer domains all at once. Once you start the transfer of your domain names you'll see the incoming requests to transfer in your DreamHost panel. More details on https://help.dreamhost.com/hc/en-us/articles/215250318-How-do-I-transfer-my-domain-registration-to-DreamHost-
You sure can do that, but you'll have to do it on your own: DreamHost doesn't provide an automatic tool for mailbox migration.
It depends on where you want that package to be installed, and what the package is. On shared hosting and on VPS users don't have root privileges so only packages that can be relocated to user's home directory and don't require root are possible. On DreamCompute cloud servers you can install anything you want.
Hope this helps.