I'm not sure a discovery queue should go away completely, although it could get absorbed into a Platforms/Machines/Feeds view that showed you the discovered servers for each, and which had been imported and which had not. For example. Maybe have certain server types set for auto import. Once imported I think monitoring would start.
I like the overall flow you show.
A comment from past experiences with other products and previous RHQ/JON experiences: normally, an agent is installed configured as part of the OS layer, via an .rpm or similar of other platforms.
So, from flow perspective, I would expect first a "ping"/"active" machine you want to add, if not responding/present, then offer him to download agent.
Is there a possibility that an agent is installed and "hibernated" until a server makes that ping with initial order of monitoring ?