You can’t send the notification to an iOS app from a federated server. All notifications to apps on iOS must come via APNS and be delivered to APNS via mTLS with a client cert issued only to the developer/publisher of that one app. Each federated instance would need its own app.
Yes. Same for FCM/Android. It's not ideal, and it's not XMPP's shortcoming, but it's livable since we don't need to reveal the who or the what. And thankfully it's up to the individual user to balance their desire for automated notifications on mobile devices against their need to keep Google and Apple out of band.
