Are SKAdNetwork conversion events sent from the client and not S2S?

Seems like there is a lot of conflated vocab here. By technical definitions, you can only send SKAdNetwork Conversion Values (to Apple) from the client. Apple doesn't receive any "Events" in the way that most analytics and attribution companies use that word. What mostly needs to be taken care of, if you are mapping analytic "events" to Conversion Values, has to be taken care of on the client and recorded server side for optimization later.

I'm having this problem as well. More specifically, to qualify "free trials" better, I'd like to declare in SKAN conversion values the event "has started the trial but not cancelled within 20minutes" (or send both events separately, doesn't matter). The problem being that those cancellations events aren't recorded by the app itself but third party tools which sync though S2S.

I wonder if there's a way to send back that event to the app, and have its conversion value processed there, but it seems quite unreliable. If you find a way keep us posted!

Yes. Per Apple's documentation:

When a user installs and launches an app as a result of ad network’s ad, the app is responsible for calling registerAppForAdNetworkAttribution() or updateConversionValue(_:). These calls prepare the install validation notification that the device sends to the ad network’s registered postback URL.