Off the top of my head, I'm thinking we could

• update BaseElement::CMSEditLink to return null when the ElementalArea is attached to an object without a CMSEditLink` method.
• Implement a way for a DataObject to explicitly say, "go through this ModelAdmin to edit me"

Related/ duplicate: #718

Possibly related: #965

I suggest to add a few changes in the CMSEditLink() method in class BaseElement.

        if (!$page instanceof SiteTree) { 
            if (method_exists($page, 'CMSEditLink')) {
                $link = Controller::join_links($page->CMSEditLink(), 'ItemEditForm');
            } else {
                $link = null;
                $this->extend('updateCMSEditLink', $link);
                return $link;
            }
        } else {
            $link = $page->CMSEditLink();
        }

1. Move checking existing CMSEditLink method in separate "IF condition".
2. If method doesn't exist return null. Effect of this approach "Page not found", since CMS user didn't override method. Since if we check method if (!$page instanceof SiteTree && method_exists($page, 'CMSEditLink')) and evoke method in "ELSE condition", it trows the error that is not displayed on screen (available only in browser console), so CMS user cannot understands what happens.
3. Other solution, we don't change this block and leave as it is. But CMS user has to define CMSEditLink in his Model class or implement CMSPreviewable interface and must have CMSEditLink in this case.

Other block of code, that I suggest to improve:

        if (!$this->inlineEditable() || $directLink) {
            if (!$page instanceof SiteTree) {
                $link = Controller::join_links($link,
                    'field',
                    $relationName,
                    'item',
                    $this->ID,
                    'edit'
                );
            } else {
                $link = Controller::join_links(
                    singleton(CMSPageEditController::class)->Link('EditForm'),
                    $page->ID,
                    'field/' . $relationName . '/item/',
                    $this->ID,
                    'edit'
                );
            }
        }     

1. Add checking that $page is not a Page instance. That will allow to define the special link for Element in ElementalArea in DataObject and load Element edit page. I've done tests. This works with ElementalContent, Elemental UserForm and Custom elements.

I'm going to provide an instruction how to use Elemental Area in DataObject and how to display these Elements on a Page in section "Model and DataBases. How To's " in Developer Guides

Looks good ... I think those suggested improvement are a nice touch. When checking if a object has a method, use ClassInfo::hasMethod instead of the native method_exists PHP method. ClassInfo::hasMethod will account for things like extensions and so on.