About your first issue, would you use two screens? If it's the case, doesn't the program behave better if you close the 3D view window after moving it on the same screen as Sweet Home 3D main window?
Otherwise, you could attach the 3D view to the main window by removing the line in the file Home.xml found in the SH3D zip file (be sure to backup your SH3D file before trying to manually edit it). If it's too complicated for you, just send it to [url=mailto:[email protected]]me[/url], I'll fix it.

About the second issue, go the folder Library/Containers/com.eteks.sweethome3d.SweetHome3D/Data/Library/Application Support/eTeks/Sweet Home 3D/recovery of your user folder and remove the files you'll find there. An easy way to reach that folder is too choose the Go > Go To Folder menu item in the Finder and copy-paste ~/Library/Containers/com.eteks.sweethome3d.SweetHome3D/Data/Library/Application Support/eTeks/Sweet Home 3D/recovery (with the leading tilde) in the prompted dialog box.