Ciao!
About the agents, try to see in the export_descr builsings about the agent_limit entry (to stimulate more; you can increase the limit for the AI factions using the event_counter that you see in the campaign_script, the one putting conditions for whether the Human or the AI). moreover, you could decrease the recruit_priority_offset of units in the export_descr_units, so there will be higher attitude towards agents.
For the techs, the Byzantines start better than others, hence you'll have to wait more to unlock new ones.
For the message events Byzantium should have the same scripts as the others in the c_s, I'll need to look at it...