Do you have a screenshot of the debugger session you can share?  Or from the Assurance feature?

Did you create the XDM Schema & the corresponding dataset?  Check in the ingestion & validation on the dataset to see if there is any data coming in or if there are any errors listed. 

Take a look at this thread: https://experienceleaguecommunities.adobe.com/t5/adobe-analytics-questions/iframe-load-event-adobe-data-collection/td-p/593708   It might be helpful.  Working with iframes is very challenging and quite frankly can be unstable.  A solution that works today could have issue...

That is the fickle nature of front end web interactions.  There is so much in play with content on the page, what js might be building the breadcrumbs, are there are factors in play that modify the CSS as the page loads to format the breadcrumbs, etc, etc, etc, etc.        Within add the delay you c...

Have you resolved this?    Here is a more updated documentation on updating attributes that might be helpful as well: https://experienceleague.adobe.com/docs/platform-learn/tutorials/profiles/update-a-specific-attribute-with-upsert.html?lang=en    

Can you provide more context?  What uses cases are you trying to solve for here?   As far as APIs go though, https://developer.adobe.com/experience-platform-apis/ here is a great place to start for getting more information on working with the Adobe Experience Platform with our various API's.

I would start here:  https://experienceleague.adobe.com/docs/experience-platform/edge/data-collection/adobe-analytics/analytics-overview.html?lang=en   Especially these sections:   XDM field group To make it easier to capture the most common Adobe Analytics metrics, we provide an Analytics field gro...

I did some testing and you need to make sure you have a Partner ID Namespace created first.  This class seems to be restricted to which type of namespaces you can use.  

https://experienceleague.adobe.com/docs/experience-platform/identity/guardrails.html?lang=en The guardrails for the identity service have been updated."When a graph with 50 linked identities is updated, Identity Service will apply a “first-in, first-out” mechanism and deletes the oldest identity to ...

Agreed.  Try the delay in the click event as suggested.