WebViewers only work for first record, so you can not get data form second or later portal row to work on that portal.
Only way to make that happen is save the key for that portal row record to a global field and then have a relationship from that global field to same related table as portal (but another name). With your new relationship you can then show data for that record and therefore it works to show WebViewer material from record 2 and forward.
Thank you for the description of an alternative! I found an approach with scripting the opening of a popover, Script triggers | A Filemaker Miscellany, that should be a good way to implement it. (If you have a more elegant way, I'm all ears.) Thanks again.