Since you are not using "Marketing emails" is the strictly HubSpot defined sense, I think I would go back to my initial suggestion of logging emails from your application as Engagements.
Seems to me to be a lot of reinvention of the wheel when you are using HubSpot as a marketing automation tool, but I'm sure you have your reasons.
Marketing emails sent by HubSpot are self instrumented - you get the timeline of events automatically recorded. The association is basically set as which contact(s) you sent the email to. This API is to help programmatically create and manage these emails, not augment the reporting.
So I'm not sure I undertstand what you are really looking to program through an API here.
right, I want to be more precise in what I would like to achieve: I have my own external email marketing service (not natively integrated) and would like to insert, via a public app, conversion events (sent, open, click) within the contacts in Hubspot. I don't know whether the feature is only available at this point if the emails are sent directly into hubspot.
Since you are not using "Marketing emails" is the strictly HubSpot defined sense, I think I would go back to my initial suggestion of logging emails from your application as Engagements.
Seems to me to be a lot of reinvention of the wheel when you are using HubSpot as a marketing automation tool, but I'm sure you have your reasons.
I did try it, but the fact is that with this type of engagement you can only associate a standard email and not a marketing email with a timeline of actions...