1. This is from the perspective of the agent. We talk about active or passive agents but, in fact, there is only one agent that we configure to be active or passive or both. Items can then be configured for active or passive.
2. No, remember it only works for passive items and there is also the configuration cache that needs to be updated first.
3. This is a possible solution and it was needed before but, since Zabbix 4.0, we now have an item, net.if.total, that calculates the total throughput of our data over the interface.