So normally, there should be a reply-to header so mails in answer of a share should already be sent to the person who shared it. I’m not sure about the bounce, but it is likely to go back to the sender. Problem with the sender, not all servers are supposed to send mails for any sender domain they are not responsible for. E.g. one of your users has a gmail address, now the invitation is send with my.user@gmail.com, however other mail server don’t expect mails from gmail coming from your server except for spam. So it’s not really an option to use the user’s mail as a sender address in general.
Perfect solution for me would be, that we encode the sender address (no-reply-message-id-123332@example.com) and Nextcloud can check the bounces and uses notifications to pass the information.