If your form is specific to these types of edits, meaning it's only used by this button, you could direct them to the page with the url I specified above (a=er&dfid=...etc) and then use form rules to set the desired field values. Since it looks like you're using hard data or "current user" you can set in form rules.
Ie. If "field 57" is empty set the field to "current user"
And
If "field 313" is empty or not Assigned set to the value "Assigned".
It would look and feel like the fields were auto populated and ensure they are set to the right value even through to saving. It just doesn't work in a grid edit fashion, but that doesn't seem to be used in this workflow.
