So, I created a custom new form for a List with SharePoint Designer 2010. But now other users can't view it. I can see it, but I don't even have permissions to change its permissions. How can I work around this and fix the permissions for the custom form?
You don't really have custom permissions for the basic list forms (new/edit/view). Those permissions are driven by the base list permissions, ie, 'New' is restricted to only those that can add items to the list. What you do have is the notion of a 'default' form for each action. If you create a custom form that you want to always use, you must set it as the 'default' form for that action (you can do this in SharePoint Designer). SharePoint will only auto-generate links to the default forms for view/edit/new. If you only want certain scenarios to use your custom forms, you must direct link to them.