Rather than leave this hanging out here unresolved, I will attempt to answer my own question, but please keep in mind that I have yet to test it out, because the capability of adding image attachments is no longer required for this particular project. However, if inquiring minds would like to know I will share how this might be accomplished in Integromat. It appears that all the attachment attribute data needed to build a functioning URL to allow an attachment to stream-in does not get pushed to the webhooks payload by default, or at least I couldn't find it even with all the Survey123 webhooks "Event data" items checked on. Maybe I missed something, but I didn't see a way to construct a dynamic URL directly from the payload. So, perhaps the better way to make this work is to use Integromat's HTTP module to leverage specific calls using the ArcGIS REST API.
Again I have not tested this, but in my example from the OP above you would need to do two HTTP requests: 1) generate a token, and 2) get the specified dynamic parts of the attachment URL. Once those pieces have been retrieved then in the Elado one could create the image URL and use the Integromat Iterator to add images if there were more than one to add. Additionally, you would want to add repeats in the Elado template maker to handle multiple images as well. I am a little unsure about how to get the attachment ID numbers, but I did see that in the webhooks payload both GlobalIDs and parentGlobalIDs come through for the attachments, so that could be a possible way to get the relevant attachment IDs for the URL. Also, there is an UploadID field, which might prove useful in getting an answer. When all is said and done you should have a URL resembling the following below.
On paper, this seems to make sense, but again the challenge will be to get the right attachment IDs that are relevant to the correct images.