I understand this helps to cover short term disconnects (switching between access points, switching from WiFi to mobile connection, etc.) but I would not really call it "offline" messages, unless both participants can get offline for at least a couple hours at a time.

What's the main problem with this ? DoS potential (users flooding the network with events/node storage exhaustion ?).

I guess UI could kinda work around this to a degree by showing if a given message was delivered yet or not, right ? It would not be good if messages could just vanish without any side being notified about that.