Is the cheat if you want to be able to add records in grid edit. Or if you're worried that users might add records and grid edit. The first thing is that you should make related parent a required field so that there's no chance of creating an orphan. But that still doesn't solve the problem that you don't want to have the user having to choose the related parent. So one way to do that is to make a button that will display a super mini version on the legacy form. They were just be a couple fields at the top to identify the parent and then you would have the embedded report. The button would open Record in edit mode and specify that particular record ID. So that way the users can add records on the legacy form and they will not be orphans.
URLRoot() & "db/" & dbid() & "?a=er&rid=" & [Record ID#]
& "&dfid=99" // replace 99 with the super mini Legacy form ID.