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I've answered this question in deep detail a few times. Once just a few days ago, I believe. So please do a deep search in the Community and post back if you need clarification.
I'd dismiss immediately using the REST API in response to individual user activities. It is not suitable for this task because it is designed for bulk and/or strictly metered use. The Munchkin API, yes.
Note that 3rd-party "opens & deliveries" cannot be logged as Marketo Delivered and Opened activities because the email assets do not exist in Marketo. External events must be logged as synthetic Click or Visit activities, or (far less scalably) as custom activities.
Thanks Sanford. I'll dig a little deeper. I was also considering using Mixpanel to track the high level opens.