1) if the only change being made is to the CSS, the CSS file for the form is in the webassist/forms folder. Dreamweaver will show the names of linked files along the top to help you pinpoint the exact files, if in double, upload the entire webassist folder.
2) Yes, when making CSS Changes, form builder needs to be able to edit the linked CSS Files, if checkout / Checkin is enabled, form builder wont be able to make those changes if the CSS file is not checked out.