This document gives a good overview of which Recommendations parameters are supported by Web SDK, as well as an example at the bottom of how to include them in the sendEvent request

You could detect the OS by checking the User Agent and then use some conditional JS, based on the outcome of that check, to render the correct the image accordingly 

You should be able to edit the HTML directly and replace the background-image attribute; alternatively, you could choose select the div, choose Edit > Styles from the VEC options and then add in the image source there

Great, that's working as expected then, at least in terms of the two interact requests. Typical latency is around 4mins for the metric to update in the reporting UI

You wouldn't have an empty activity with that mbox name, you would just add a metric - with the definition of mbox name = myMboxName - to your real activity in the Goals & Settings section. Like this:  

With it being an Analytics segment, it's relying on targeting based on ECIDs within that AA segment membership. If customers aren't visiting regularly, use a different browser, use incognito mode etc then they will have a new ECID, which would mean you could lose a lot of the historical segment memb...

The personalization:decisions payload from Target is empty, which implies that there is no match in for the mbox name (decisionScope) that was sent in the request. Is the activity live in Target?

Could you share what's in the response payload from your first sendEvent request? e.g. should look something like.... 

Ok, and have you added conversionTest as the metric in the Goals & Settings section of the activity? 

Yes, correct you should see a second call, could you share the code that you're using?The mbox doesn't have to be called "conversionTest", you can name it anything; you can just type it into the field even if it doesn't already exist, so I can add a new mbox called newMboxName and it's not a problem...