The state you’re looking for doesn’t exist unless you create it. It needs to be in the form of an object in a data view. So, I recommend creating a non-persistent helper entity that contains a boolean.
The proper answer is you cannot easily do what I wanted to. You cannot use a helper object for a popup window triggered from a form input control to use as a picker for a list of items, as the return value of the popup must match the form field type expected.
The best option is to have one page as a popup and copy-paste it, then change the duplicate. Having two duplicates is not ideal, but it works for now. The benefit at least is I can customize them more individually (one as a popup and one with a layout). It would have been great if snippets could be use in this case. I don’t know why a page cannot detect that a data grid exists in a snippet, or just “hand off” its context to a snippet on the page (a forward option), so I don’t have to maintain multiple copies of a data grid, but perhaps this is an edge case. Ideally, it would be great to pass varaibles created in a microflow to a page. This does not feel very flexible at all.